Acquisitions/Dispositions Acquisitions/Dispositions - Schedule of Business Acquisitions, by Acquisition (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Assets		$ 205,962	$ 204,875
Proceeds from Divestiture of Businesses		261	0	$ 0
Goodwill, Written off Related to Sale of Business Unit		7
Gain (Loss) on Disposition of Business		107	0	$ 0
Premium Assignment Corporation [Member]
Business Acquisition [Line Items]
Assets		1,300
Proceeds from Divestiture of Businesses		261
Goodwill, Written off Related to Sale of Business Unit		7
Intangible Assets, Written off Related to Sale of Business Unit		0
Gain (Loss) on Disposition of Business		107
Pillar Financial [Member]
Business Acquisition [Line Items]
Business Combination, Consideration Transferred			(197)
Goodwill, Acquired During Period		1
Indefinite-lived Intangible Assets Acquired	[1]	$ 13
Business Combination, Bargain Purchase, Gain Recognized, Amount			$ 0

[1] Does not include $62 million of commercial mortgage servicing rights acquired.