Note 4 - Fair Value of Financial Instruments Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($)		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018	Dec. 31, 2018
Fair Value by Hierarchy Level [Line Items]
Other Investments		$ 3,059,000			$ 3,415,000
Reinvested cash collateral held under securities lending agreements		6,233,000			11,699,000
Loaned securities		22,606,000			27,860,000
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value by Hierarchy Level [Line Items]
Other Investments		3,100,000			3,400,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,495,317,000	[1],[2]		5,177,474,000	[3],[4]
Total Assets at Fair Value		5,495,317,000	[2],[5]		5,177,474,000	[4],[6]
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 | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		212,518,000			227,714,000
Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		233,827,000			324,742,000
Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		174,541,000			95,132,000
Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,485,463,000			2,564,068,000
Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		368,495,000			353,224,000
Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		549,986,000			591,393,000
Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		675,477,000			705,468,000
Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		141,105,000			140,620,000
Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		653,905,000	[7]		175,113,000	[8]
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		489,556,000	[1]		431,096,000	[3]
Total Assets at Fair Value		489,556,000	[5]		431,096,000	[6]
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		178,908,000			199,302,000
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
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		174,541,000			95,132,000
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
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0
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
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		136,107,000			136,662,000
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0	[7]		0	[8]
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		5,005,761,000	[1]		4,746,378,000	[3]
Total Assets at Fair Value		5,005,761,000	[5]		4,746,378,000	[6]
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		33,610,000			28,412,000
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		233,827,000			324,742,000
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
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		2,485,463,000			2,564,068,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		368,495,000			353,224,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		549,986,000			591,393,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		675,477,000			705,468,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,998,000			3,958,000
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		653,905,000	[7]		175,113,000	[8]
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Assets at Fair Value		0			0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		0			0
Securities Financing Transaction, Fair Value
Fair Value by Hierarchy Level [Line Items]
Reinvested cash collateral held under securities lending agreements	[9]	17,372,000			16,815,000
Loaned securities	[10]	22,606,000			27,860,000
Securities Financing Transaction, Fair Value | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[10]	6,526,000			7,818,000
Securities Financing Transaction, Fair Value | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities	[10]	$ 16,080,000			$ 10,055,000

[1] Does not include certain other invested assets of $3.1 million that are primarily invested in limited partnership investments valued using the net asset value as a practical expedient. Includes cash collateral held under securities lending agreements of $6.2 million that is reinvested in money market instruments.

[2] Includes $22.6 million of securities loaned to third-party Borrowers under securities lending agreements, classified as other assets in our condensed consolidated balance sheets. See Note 5 for more information.

[3] Does not include certain 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. Includes cash collateral held under securities lending agreements of $11.7 million that is reinvested in money market instruments.

[4] Includes $27.9 million of securities loaned to third-party Borrowers under securities lending agreements, classified as other assets in our condensed consolidated balance sheets. See Note 5 for more information.

[5] Does not include the fair value of an immaterial embedded derivative, which we have accounted for separately as a freestanding derivative and classified in other assets in our condensed consolidated balance sheet. See Note 7 for more information.

[6] Does not include the fair value of an immaterial embedded derivative, which we have accounted for separately as a freestanding derivative and classified in other assets in our condensed consolidated balance sheet. See Note 7 for more information.

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

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

[9] Securities collateral on deposit with us from Borrowers may not be transferred or re-pledged unless the Borrower is in default, and is therefore not reflected in our condensed consolidated financial statements.

[10] Our securities loaned under securities lending agreements are reported at fair value within other assets in our condensed consolidated balance sheets. All of our securities lending agreements are classified as overnight and revolving. None of the amounts are subject to offsetting.