Investments in Unconsolidated Affiliates (Financial Information) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			1 Months Ended	9 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2007	Sep. 30, 2013	Dec. 31, 2011
Symetra income statement data: [Abstract]
Equity in earnings of unconsolidated affiliates, net of tax	$ 12.3	$ 7.0	$ 12.5	$ 13.8	$ 11.7	$ 8.6	$ 7.1	$ 9.2	$ 45.6	$ 36.6	$ 29.9
Investments in unconsolidated affiliates	414.4				321.4				414.4	321.4
Symetra
Symetra balance sheet data; [Abstract]
Total investments	30,634.3				27,901.1				30,634.3	27,901.1
Separate account assets	949.8				978.4				949.8	978.4
Total assets	32,994.2				30,129.5				32,994.2	30,129.5
Policyholder liabilities	27,276.0				25,328.8				27,276.0	25,328.8
Long-term debt	697.2				449.5				697.2	449.5
Separate account liabilities	949.8				978.4				949.8	978.4
Total liabilities	29,633.6				27,187.6				29,633.6	27,187.6
Common shareholders’ equity	3,360.6				2,941.9				3,360.6	2,941.9
Symetra income statement data: [Abstract]
Net premiums earned									629.1	627.2	605.0
Net investment income									1,320.5	1,285.0	1,275.2
Total revenues									2,182.3	2,103.9	2,101.2
Policy benefits									1,399.6	1,394.9	1,371.8
Total expenses									1,882.4	1,865.3	1,831.1
Net income									254.4	220.7	205.4
Comprehensive net income (loss)									397.0	(777.6)	549.3
Ownership interest (as a percent)	17.31%				17.03%				17.31%	17.03%
Equity in earnings of unconsolidated affiliates, net of tax									47.0	37.8	32.3
Dividends received									(34.1)	(6.4)	(4.9)
Investments in unconsolidated affiliates	411.4				317.3				411.4	317.3	381.5			273.6
Pentelia Capital Management
Symetra income statement data: [Abstract]
Ownership interest (as a percent)												33.00%
Payments to acquire investment												1.6
Equity in earnings of unconsolidated affiliates, net of tax									0.1	(1.3)
Dividends received									(0.4)
Hamer, LLC
Symetra income statement data: [Abstract]
Equity in earnings of unconsolidated affiliates, net of tax									1.9	0.9	0.4
Dividends received									(3.0)	(0.8)
Investments in unconsolidated affiliates	3.0				4.1				3.0	4.1
Bri-Mar Manufacturing, LLC
Symetra income statement data: [Abstract]
Equity in earnings of unconsolidated affiliates, net of tax													0.9
Proceeds from Sale of Equity Method Investments										1.1
Equity Method Investment, Realized Gain (Loss) on Disposal										1.7
Total | Symetra
Symetra income statement data: [Abstract]
Equity in earnings of unconsolidated affiliates, net of tax									47.0	37.8	32.3
Dividends received									(34.1)	(6.4)	(4.9)
Investments in unconsolidated affiliates	411.4				317.3				411.4	317.3	351.2			261.0
Per share price, class of warrant (in dollars per share)	$ 23.05								$ 23.05
Total | Amortization of common share basis difference [Member] | Symetra
Symetra income statement data: [Abstract]
Equity in earnings of unconsolidated affiliates, net of tax									$ 3.0	$ 3.0