Note 5 - Fair Value of Financial Instruments - Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	$ 130,850,000	$ 130,850,000	$ 57,499,000
Other Investments	3,624,000	3,624,000	4,973,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	6,681,035,000	6,681,035,000	6,786,469,000
Derivative, Fair Value, Net	5,905,000	5,905,000	5,513,000
Assets, Fair Value Disclosure	6,817,790,000	6,817,790,000	6,849,481,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	775,124,000	775,124,000	551,695,000
Derivative, Fair Value, Net	0	0	0
Assets, Fair Value Disclosure	810,656,000	810,656,000	577,870,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,902,911,000	5,902,911,000	6,231,774,000
Derivative, Fair Value, Net	0	0	0
Assets, Fair Value Disclosure	5,998,229,000	5,998,229,000	6,263,098,000
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	3,000,000	3,000,000	3,000,000
Derivative, Fair Value, Net	5,905,000	5,905,000	5,513,000
Assets, Fair Value Disclosure	8,905,000	8,905,000	8,513,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	5,550,000	5,550,000	4,876,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	95,318,000	95,318,000	31,324,000
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	29,982,000	29,982,000	21,299,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	5,550,000	5,550,000	4,876,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	29,982,000	29,982,000	21,299,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	95,318,000	95,318,000	31,324,000
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	0	0	0
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Less: loaned securities	0	0	0
Fixed-maturities available for sale | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	213,954,000	213,954,000	176,033,000
Fixed-maturities available for sale | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	141,431,000	141,431,000	149,258,000
Fixed-maturities available for sale | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	2,909,453,000	2,909,453,000	2,832,350,000
Fixed-maturities available for sale | RMBS
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	701,664,000	701,664,000	799,814,000
Fixed-maturities available for sale | CMBS
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	685,595,000	685,595,000	645,071,000
Fixed-maturities available for sale | CLO
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	519,909,000	519,909,000	569,173,000
Fixed-maturities available for sale | Other ABS
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	225,012,000	225,012,000	249,988,000
Fixed-maturities available for sale | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	5,105,000	5,105,000	5,100,000
Fixed-maturities available for sale | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis, Including Loaned Securities	45,384,000	45,384,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,572,813,000	5,572,813,000	5,723,340,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	208,663,000	208,663,000	169,223,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	157,944,000	157,944,000	165,271,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	2,970,142,000	2,970,142,000	3,047,189,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	719,840,000	719,840,000	833,939,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	714,671,000	714,671,000	681,265,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	521,153,000	521,153,000	568,558,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	226,968,000	226,968,000	252,457,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,399,000	5,399,000	5,438,000
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	48,033,000	48,033,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	178,709,000	178,709,000	140,034,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	178,709,000	178,709,000	140,034,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,394,104,000	5,394,104,000	5,583,306,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	29,954,000	29,954,000	29,189,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	157,944,000	157,944,000	165,271,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	2,970,142,000	2,970,142,000	3,047,189,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	719,840,000	719,840,000	833,939,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	714,671,000	714,671,000	681,265,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	521,153,000	521,153,000	568,558,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	226,968,000	226,968,000	252,457,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	5,399,000	5,399,000	5,438,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	48,033,000	48,033,000
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Fixed-maturities available for sale | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Trading Securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	267,980,000	267,980,000	290,885,000
Trading Securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	101,454,000	101,454,000	120,449,000
Trading Securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	121,377,000	121,377,000	123,142,000
Trading Securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	11,134,000	11,134,000	13,000,000
Trading Securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,015,000	34,015,000	34,294,000
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	267,980,000	267,980,000	290,885,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	101,454,000	101,454,000	120,449,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	121,377,000	121,377,000	123,142,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	11,134,000	11,134,000	13,000,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	34,015,000	34,015,000	34,294,000
Trading Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Trading Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Equity securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	175,147,000	175,147,000	151,240,000
Equity securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	167,610,000	167,610,000	142,761,000
Equity securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	7,537,000	7,537,000	8,479,000
Equity securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	662,095,000	662,095,000	618,004,000
Short-term investments | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	21,997,000	21,997,000
Short-term investments | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	15,940,000	15,940,000	21,819,000
Short-term investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	406,808,000	406,808,000	268,900,000
Short-term investments | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	19,931,000	19,931,000	30,495,000
Short-term investments | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	4,780,000	4,780,000
Short-term investments | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	431,000	431,000	219,000
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	192,208,000	192,208,000	296,571,000
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	428,805,000	428,805,000	268,900,000
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	21,997,000	21,997,000
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	406,808,000	406,808,000	268,900,000
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	233,290,000	233,290,000	349,104,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	15,940,000	15,940,000	21,819,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	19,931,000	19,931,000	30,495,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	4,780,000	4,780,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	431,000	431,000	219,000
Short-term investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	192,208,000	192,208,000	296,571,000
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | U.S. government and agencies securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Short-term investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	3,000,000	3,000,000	3,000,000
Other investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Other investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	0	0	0
Other investments | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	3,000,000	3,000,000	3,000,000
Partnership investment | Investments Measured at NAV
Fair Value by Hierarchy Level [Line Items]
Other Investments	$ 600,000	$ 600,000	$ 2,000,000.0