Note 4 - Fair Value of Financial Instruments Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)		6 Months Ended		12 Months Ended
		Jun. 30, 2019		Dec. 31, 2018
Fair Value by Hierarchy Level [Line Items]
Loaned securities		$ 29,064,000		$ 27,860,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[1]	5,510,501,000
Assets, Fair Value Disclosure		5,539,565,000	[1]	5,177,474,000	[2]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0		0
Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[2]			5,149,614,000
Fair Value, Measurements, Recurring | Investments Measured at NAV
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,800,000		3,400,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[1]	599,483,000
Assets, Fair Value Disclosure		605,536,000	[1]	431,096,000	[2]
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	[2]			411,054,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[1]	4,911,018,000
Assets, Fair Value Disclosure		4,934,029,000	[1]	4,746,378,000	[2]
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]			4,738,560,000
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Assets, Fair Value Disclosure		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		0		0
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		1,502,000	[3]	9,987,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities		23,011,000	[3]	7,818,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		4,551,000	[3]	10,055,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		1,502,000	[3]	9,987,000	[4]
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]
Loaned securities		0	[3]	0	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		4,551,000	[3]	10,055,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		0	[3]	0	[4]
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]
Loaned securities		23,011,000	[3]	7,818,000	[4]
Securities Financing Transaction, Fair Value | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities		0	[3]	0	[4]
Fixed maturities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,113,650,000		4,021,575,000
Fixed maturities | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		84,420,000		84,070,000
Fixed maturities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		107,962,000		138,313,000
Fixed maturities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,249,315,000		2,222,473,000
Fixed maturities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		433,773,000		332,142,000
Fixed maturities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		571,923,000		539,915,000
Fixed maturities | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		661,089,000		704,662,000
Fixed maturities | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,168,000
Fixed maturities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		49,929,000		55,658,000
Fixed maturities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		49,929,000		55,658,000
Fixed maturities | 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
Fixed maturities | 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
Fixed maturities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Fixed maturities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0		0
Fixed maturities | 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
Fixed maturities | 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
Fixed maturities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,063,721,000		3,965,917,000
Fixed maturities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		34,491,000		28,412,000
Fixed maturities | 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		107,962,000		138,313,000
Fixed maturities | 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		2,249,315,000		2,222,473,000
Fixed maturities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		433,773,000		332,142,000
Fixed maturities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		571,923,000		539,915,000
Fixed maturities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		661,089,000		704,662,000
Fixed maturities | 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		5,168,000
Trading Securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		337,017,000		469,071,000
Trading Securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		128,824,000		168,359,000
Trading Securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		155,793,000		228,152,000
Trading Securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		17,511,000		21,082,000
Trading Securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		34,889,000		51,478,000
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
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
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
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
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
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		337,017,000		469,071,000
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		128,824,000		168,359,000
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		155,793,000		228,152,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		17,511,000		21,082,000
Trading Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		34,889,000		51,478,000
Equity securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		149,954,000		130,565,000
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		144,882,000		126,607,000
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		5,072,000		3,958,000
Short-term Investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		909,880,000		528,403,000
Short-term Investments | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		142,206,000		133,657,000
Short-term Investments | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		13,990,000		18,070,000
Short-term Investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		262,466,000		95,132,000
Short-term Investments | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		89,910,000		105,625,000
Short-term Investments | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value				806,000
Short-term Investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		401,308,000	[5]	175,113,000	[6]
Short-term Investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		404,672,000		228,789,000
Short-term Investments | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		142,206,000		133,657,000
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
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		262,466,000		95,132,000
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
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
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	[5]	0	[6]
Short-term Investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		505,208,000		299,614,000
Short-term Investments | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | US government and agency 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		13,990,000		18,070,000
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
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		89,910,000		105,625,000
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				806,000
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		$ 401,308,000	[5]	$ 175,113,000	[6]

[1] Does not include other invested assets of $2.8 million that are primarily invested in limited partnership investments valued using the net asset value as a practical expedient.

[2] Does not include other invested assets of $3.4 million that are primarily invested in limited partnership investments valued using the net asset value as a practical expedient.

[3] Securities loaned to third-party borrowers under securities lending agreements are classified as other assets in our condensed consolidated balance sheets. See Note 5 for more information.

[4] Securities loaned to third-party borrowers under securities lending agreements are classified as other assets in our condensed consolidated balance sheets. See Note 5 for more information.

[5] Comprising short-term certificates of deposit and commercial paper.

[6] Comprising short-term certificates of deposit and commercial paper.