Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 581,700,000	$ 606,800,000
Included in Condensed Consolidated Statement of Operations	(4,500,000)	(4,500,000)
Included in Other Comprehensive Income (Loss)	(2,500,000)	(5,500,000)
Purchases	40,100,000	41,800,000
Settlements	(22,800,000)	(6,200,000)
Sales	(29,900,000)	(38,000,000)
Transfers out of Level 3	(10,800,000)	0
Balance at End of Period	551,300,000	594,400,000
Fair Value, Level 1 to level 2 Transfers, Amount	0	0
Fair Value, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Fair Value, Level 1 to level 3 Transfers, Amount	0	0
Fair Value, Level 3 to level 1 Transfers, Amount	0	0
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	403,200,000	436,300,000
Included in Condensed Consolidated Statement of Operations	(3,300,000)	(2,500,000)
Included in Other Comprehensive Income (Loss)	1,400,000	(800,000)
Purchases	29,800,000	40,100,000
Settlements	(15,000,000)	(5,800,000)
Sales	(22,000,000)	(35,500,000)
Transfers out of Level 3	(2,900,000)
Balance at End of Period	391,200,000	431,800,000
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	3,800,000
Included in Condensed Consolidated Statement of Operations	(1,200,000)
Included in Other Comprehensive Income (Loss)	0
Purchases	0
Settlements	0
Sales	0
Transfers out of Level 3	0
Balance at End of Period	2,600,000
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	600,000	3,800,000
Included in Condensed Consolidated Statement of Operations	0	0
Included in Other Comprehensive Income (Loss)	100,000	(100,000)
Purchases	0	0
Settlements	(500,000)	(400,000)
Sales	0	0
Transfers out of Level 3	0
Balance at End of Period	200,000	3,300,000
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	103,500,000	87,300,000
Included in Condensed Consolidated Statement of Operations	200,000	(600,000)
Included in Other Comprehensive Income (Loss)	1,900,000	(5,200,000)
Purchases	9,200,000	1,700,000
Settlements	(7,300,000)	0
Sales	(3,800,000)	0
Transfers out of Level 3	(7,900,000)
Balance at End of Period	95,800,000	83,200,000
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		3,800,000
Included in Condensed Consolidated Statement of Operations		0
Included in Other Comprehensive Income (Loss)		(200,000)
Purchases		0
Settlements		0
Sales		0
Balance at End of Period		3,600,000
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	29,700,000	30,000,000
Included in Condensed Consolidated Statement of Operations	(200,000)	(1,000,000)
Included in Other Comprehensive Income (Loss)	700,000	1,300,000
Purchases	300,000	0
Settlements	0	0
Sales	(1,400,000)	0
Transfers out of Level 3	0
Balance at End of Period	29,100,000	30,300,000
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	40,900,000	45,600,000
Included in Condensed Consolidated Statement of Operations	0	(400,000)
Included in Other Comprehensive Income (Loss)	(6,600,000)	(500,000)
Purchases	800,000	0
Settlements	0	0
Sales	(2,700,000)	(2,500,000)
Transfers out of Level 3	0
Balance at End of Period	$ 32,400,000	$ 42,200,000