Acquisitions and Divestitures (AmRisc-American Coastal Summary) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Purchase of Additional Ownership of AmRisc, LP
Fair value of American Coastal	$ 216
Cash paid	146
Total consideration	362
Deferred tax asset recognized	(140)
Net charge to shareholders' equity	222
Sale of American Coastal
Fair value of American Coastal	216
Net assets sold of American Coastal	(193)
Allocated goodwill	(49)
Pre-tax loss on sale of American Coastal	(26)
Income tax expense on sale of American Coastal	(8)
After tax net loss on sale of American Coastal	$ (34)