Note 4 - Fair Value of Financial Instruments Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($)		3 Months Ended			6 Months Ended
		Jun. 30, 2015		Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014	Dec. 31, 2014
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value by Hierarchy Level [Line Items]
Other Invested Assets, Carrying Amount		$ 13,600,000			$ 13,600,000			$ 14,600,000
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		4,295,500,000	[1]		4,295,500,000	[1]		3,614,700,000	[2]
Total Assets at Fair Value		4,295,500,000			4,295,500,000			3,614,700,000
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
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0			0
US government and agency securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		598,300,000			598,300,000			839,900,000
State and municipal obligations | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		330,800,000			330,800,000			362,800,000
Money market instruments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		828,100,000			828,100,000			600,300,000
Corporate bonds and notes | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		1,321,700,000			1,321,700,000			992,800,000
RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		237,300,000			237,300,000			132,300,000
CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		383,400,000			383,400,000			246,800,000
Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		274,000,000			274,000,000			185,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				$ 17,300,000			$ 17,300,000
Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		36,700,000			36,700,000			37,700,000
Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		68,000,000			68,000,000			215,600,000	[3]
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		217,200,000	[4]		217,200,000	[4]		1,000,000	[5]
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		1,447,500,000	[1]		1,447,500,000	[1]		1,601,200,000	[2]
Total Assets at Fair Value		1,447,500,000			1,447,500,000			1,601,200,000
Assets Held-for-sale, Long Lived, Fair Value Disclosure								495,100,000
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		598,300,000			598,300,000			836,900,000
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		828,100,000			828,100,000			600,300,000
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		21,100,000			21,100,000			164,000,000	[3]
Fair Value, Inputs, Level 1 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0	[4]		0	[4]		0	[5]
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		2,847,500,000	[1]		2,847,500,000	[1]		2,013,500,000	[2]
Total Assets at Fair Value		2,847,500,000			2,847,500,000			2,013,500,000
Assets Held-for-sale, Long Lived, Fair Value Disclosure								839,200,000
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		0			0			3,000,000
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		330,800,000			330,800,000			362,800,000
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		1,321,700,000			1,321,700,000			992,800,000
Fair Value, Inputs, Level 2 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		237,300,000			237,300,000			132,300,000
Fair Value, Inputs, Level 2 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		383,400,000			383,400,000			246,800,000
Fair Value, Inputs, Level 2 | Other ABS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		274,000,000			274,000,000			185,500,000
Fair Value, Inputs, Level 2 | Foreign government securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		36,700,000			36,700,000			37,700,000
Fair Value, Inputs, Level 2 | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		46,400,000			46,400,000			51,600,000	[3]
Fair Value, Inputs, Level 2 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		217,200,000	[4]		217,200,000	[4]		1,000,000	[5]
Fair Value, Inputs, Level 2 | Commercial Paper | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		216,200,000			216,200,000
Fair Value, Inputs, Level 2 | Short Term CDs | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		1,000,000			1,000,000
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[1]	500,000			500,000
Total Assets at Fair Value		500,000			500,000			0
Liabilities, Fair Value Disclosure, Recurring		0			0
Assets Held-for-sale, Long Lived, Fair Value Disclosure								102,600,000
Variable Interest Entity Debt								$ 3,800,000
Total Level III Liabilities								100.00%
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			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			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			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			0
Fair Value, Inputs, Level 3 | RMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			0
Fair Value, Inputs, Level 3 | CMBS | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value		0			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			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			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			500,000
Total Assets at Fair Value		500,000			500,000
Gain (Loss) on Investments		0
Fair Value, Inputs, Level 3 | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total Investments at Fair Value	[4]	$ 0			$ 0
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		0.10%			0.10%

[1] Does not include certain other invested assets ($13.6 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 ($14.6 million), primarily invested in limited partnerships, accounted for as cost-method investments and not measured at fair value. Also excludes investments classified as assets held for sale of $495.1 million, $839.2 million and $102.6 million, with fair values categorized in Level I, Level II and Level III, respectively.

[3] Comprising broadly diversified domestic equity mutual funds and certain common stocks included within Level I and various preferred stocks invested across numerous companies and industries included within Level II.

[4] Comprising short-term certificates of deposit ($1.0 million) and short-term commercial paper ($216.2 million) included within Level II.

[5] Comprising short-term certificates of deposit.