CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (Parenthetical) - USD ($) shares in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Stockholders' Equity [Abstract]
Common stock issuance cost			$ 1	$ 1	$ 1
Dividends per share of common stock (in dollars per share)	$ 0.770	$ 0.725	$ 3.08	$ 2.90	$ 2.64
Outstanding and unallocated shares held by the Employee Stock Ownership Plan (ESOP) Trust (in shares)	1	1	1	1	2
Original number of shares purchased and held by the ESOP Trust (in shares)	25	25	25	25	25