Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 6.8	$ 0.0	$ 14.1
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	10,863		6,194
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	$ (0.2)	0.1	$ (0.1)	0.2
Net Income (Loss) Available to Common Stockholders, Basic	$ 341.1	$ 4.3	$ 354.1	$ 88.6
Total average common shares outstanding during the period	5,096,100	5,985,700	5,317,800	5,982,100
Average unvested restricted common shares	(66,500)	(71,400)	(60,200)	(65,200)
Basic earnings per share denominator	5,029,600	5,914,300	5,257,600	5,916,900
Average outstanding dilutive options to acquire common shares	5,107,000	5,985,700	5,324,300	5,982,100
Diluted earnings (loss) per share denominator	5,040,500	5,914,300	5,264,100	5,916,900
Earnings Per Share, Diluted	$ 66.79	$ 0.72	$ 66.50	$ 14.80
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net loss from discontinued operations, net of tax	$ 358.6	$ (0.5)	$ 359.7	$ 70.8
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	(4.7)	0.0	4.1	(0.8)
Net Income (Loss) Available to Common Stockholders, Basic	$ 353.9	$ (0.5)	$ 355.6	$ 70.0
Total average common shares outstanding during the period	5,107,000	5,985,700	5,324,000	5,982,100
Average unvested restricted common shares	(66,500)	(71,400)	(60,200)	(65,200)
Basic earnings per share denominator	5,029,600	5,914,300	5,257,600	5,916,900
Diluted earnings (loss) per share denominator	5,040,500	5,914,300	5,263,800	5,916,900
Earnings Per Share, Basic and Diluted	$ 70.37	$ (0.08)	$ 67.64	$ 11.84
Earnings Per Share, Diluted	$ 70.22	$ (0.08)	$ 67.56	$ 11.84
Common Stock [Member] | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax		$ 6.1		$ 13.3
Amortization of common share basis difference | Common Stock [Member] | Symetra
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax		$ 0.7			$ 1.4
Employee Stock Option [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	120,000		120,000