Note 4 - Fair Value of Financial Instruments Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($)		3 Months Ended			6 Months Ended			12 Months Ended
		Jun. 30, 2017		Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016	Dec. 31, 2016
Fair Value by Hierarchy Level [Line Items]
Gain (Loss) on Investments		$ 4,756,000		$ 29,265,000	$ 1,803,000		$ 58,917,000
Other than Temporary Impairment Losses, Investments		1,000,000		0	1,000,000		0
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value by Hierarchy Level [Line Items]
Cost Method Investments		783,000			783,000			$ 1,195,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,583,059,000	[1]		4,583,059,000	[1]		4,461,235,000	[2]
Total Assets at Fair Value		4,583,059,000			4,583,059,000			4,461,235,000
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		0		0	0		0
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0		0	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0		0	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0		$ 0	0		$ 0
US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		217,660,000			217,660,000			237,479,000
State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		377,437,000			377,437,000			358,536,000
Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		214,654,000			214,654,000			431,472,000
Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,231,037,000			2,231,037,000			2,024,205,000
RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		216,830,000			216,830,000			388,842,000
CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		499,368,000			499,368,000			507,273,000
Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		601,718,000			601,718,000			450,128,000
Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		36,346,000			36,346,000			32,807,000
Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		176,808,000			176,808,000			1,330,000
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		11,201,000	[3]		11,201,000	[3]		29,163,000	[4]
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		595,651,000	[1]		595,651,000	[1]		668,951,000	[2]
Total Assets at Fair Value		595,651,000			595,651,000			668,951,000
Fair Value, Inputs, Level 1 | US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		205,049,000			205,049,000			237,479,000
Fair Value, Inputs, Level 1 | State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		214,654,000			214,654,000			431,472,000
Fair Value, Inputs, Level 1 | Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 1 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		175,948,000			175,948,000			0
Fair Value, Inputs, Level 1 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0	[3]		0	[3]		0	[4]
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		3,987,408,000	[1]		3,987,408,000	[1]		3,791,284,000	[2]
Total Assets at Fair Value		3,987,408,000			3,987,408,000			3,791,284,000
Fair Value, Inputs, Level 2 | US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		12,611,000			12,611,000			0
Fair Value, Inputs, Level 2 | State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		377,437,000			377,437,000			358,536,000
Fair Value, Inputs, Level 2 | Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0			0
Fair Value, Inputs, Level 2 | Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,231,037,000			2,231,037,000			2,024,205,000
Fair Value, Inputs, Level 2 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		216,830,000			216,830,000			388,842,000
Fair Value, Inputs, Level 2 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		499,368,000			499,368,000			507,273,000
Fair Value, Inputs, Level 2 | Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		601,718,000			601,718,000			450,128,000
Fair Value, Inputs, Level 2 | Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		36,346,000			36,346,000			32,807,000
Fair Value, Inputs, Level 2 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		860,000			860,000			830,000
Fair Value, Inputs, Level 2 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		11,201,000	[3]		11,201,000	[3]		28,663,000	[4]
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[2]							1,000,000
Total Assets at Fair Value		0			0			1,000,000
Gain (Loss) on Investments								0
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		$ 0			$ 0			0
Fair Value, Inputs, Level 3 | US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								0
Fair Value, Inputs, Level 3 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value								500,000
Fair Value, Inputs, Level 3 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[4]							$ 500,000
Maximum [Member] | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Level III Assets as a Percentage of Total Assets Measured at Fair Value (less than 0.1%)								0.10%

[1] Does not include certain other invested assets ($0.8 million), primarily invested in limited partnerships, accounted for as cost-method investments and not measured at fair value.

[2] Does not include certain other invested assets ($1.2 million), primarily invested in limited partnerships, accounted for as cost-method investments and not measured at fair value.

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

[4] Comprising short-term certificates of deposit and commercial paper, included within Level II, and convertible notes of non-public company issuers, included within Level III.