Investments in Symetra (Symetra) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended					6 Months Ended
	Feb. 01, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments		$ 19.8	$ 2.0				$ 56.1	$ (250.6)
Trading securities unrealized holding gain on investment after tax		$ 20.8	0.7				53.5	(209.6)
Sales of common equity securities							$ 183.9	676.9
Common shares, par value per share (in dollars per share)		$ 1.00					$ 1.00		$ 1.00
Net realized and unrealized investment gains		$ 33.7	$ 3.4				$ 70.0	$ 16.3
Symetra
Investments in unconsolidated affiliates
Unrealized gains (losses) on investments					$ 258.8
Trading securities unrealized holding gain on investment after tax					$ 241.1
Sales of common equity securities	$ 658.0
Common shares, par value per share (in dollars per share)	$ 32.00
Net realized and unrealized investment gains				$ 4.7
Symetra
Investments in unconsolidated affiliates
Common stock, dividends, per share, cash paid	$ 0.50					$ 0.50
Sales of common equity securities	$ 658.0