CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2016	$ 11,021	$ 1,268	$ 5,881	$ 3,982	$ (110)
Balance (in shares) at Dec. 31, 2016		507,223,000
Increase (Decrease) in Stockholders' Equity
Net income	239			239
Other comprehensive income	2				2
Dividends declared on common stock	(184)			(184)
Issuances of common stock (in shares)		611,000
Issuances of common stock	5	$ 1	4
Purchase of common stock (in shares)		(71,000)
Repurchases of common stock	(3)	$ 0	(3)
Share-based compensation	(10)		(9)	(1)
Ending balance at Mar. 31, 2017	11,070	$ 1,269	5,873	4,036	(108)
Balance (in shares) at Mar. 31, 2017		507,763,000
Beginning balance at Dec. 31, 2017	$ 11,455	$ 1,269	5,898	4,413	(125)
Balance (in shares) at Dec. 31, 2017	507,762,881	507,763,000
Increase (Decrease) in Stockholders' Equity
Net income	$ 291			291
Other comprehensive income	1				1
Dividends declared on common stock	(194)			(194)
Issuances of common stock (in shares)		921,000
Issuances of common stock	17	$ 3	14
Purchase of common stock (in shares)		(22,000)
Repurchases of common stock	(1)	$ 0	(1)
Share-based compensation	(8)		(8)	0
Ending balance at Mar. 31, 2018	$ 11,561	$ 1,272	$ 5,903	$ 4,510	$ (124)
Balance (in shares) at Mar. 31, 2018	508,661,859	508,662,000