Shareholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net of tax unrealized gains (losses) related to credit-only impaired securities	$ 57	$ 57		$ 57		$ 58
Common Stock options issued under stock incentive plan, average exercise price				$ 63.15	$ 56.44
Compensation expense related to stock incentive plans	$ 7		$ 7	$ 13	$ 14
Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Common Stock, shares issued				171,130
Restricted Common Stock, fair value per share				$ 63.15
Common Stock options issued under stock incentive plan				716,818
Common Stock options issued under stock incentive plan, average exercise price				$ 63.15
Annual Equity Bonus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Other benefit plans, shares		54,732
Common Stock issued, fair value per share		$ 62.55