CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Millions	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2015	$ 10,601	$ 1,269	$ 5,889	$ 3,553	$ (110)
Balance (in shares) at Dec. 31, 2015		507,536
Increase (Decrease) in Stockholders' Equity
Net income	1,123			1,123
Other comprehensive income (loss)	0
Dividends declared on common stock	(694)			(694)
Issuances of common stock (in shares)		486
Issuances of common stock	16	$ 1	15
Stock Repurchased During Period, Shares		(799)
Repurchases of common stock	(32)	$ (2)	(30)
Share-based compensation	7		7
Ending balance at Dec. 31, 2016	$ 11,021	$ 1,268	5,881	3,982	(110)
Balance (in shares) at Dec. 31, 2016		507,223
Increase (Decrease) in Stockholders' Equity
Dividends declared on common stock	$ (1.36)
Net income	$ 1,148			1,148
Other comprehensive income (loss)	7				7
Dividends declared on common stock	(736)			(736)
Issuances of common stock (in shares)		611
Issuances of common stock	5	$ 1	4
Stock Repurchased During Period, Shares		(71)
Repurchases of common stock	(3)	$ 0	(3)
Share-based compensation	13		16	(3)
Adoption of ASU No. 2018-02	0			22	(22)
Ending balance at Dec. 31, 2017	$ 11,455	$ 1,269	5,898	4,413	(125)
Balance (in shares) at Dec. 31, 2017		507,763
Increase (Decrease) in Stockholders' Equity
Dividends declared on common stock	$ (1.44)
Net income	$ 1,261			1,261
Other comprehensive income (loss)	1				1
Dividends declared on common stock	(780)			(780)
Issuances of common stock (in shares)		6,296
Issuances of common stock	270	$ 16	254
Repurchases of common stock (in shares)		(22)
Repurchases of common stock	(1)	$ 0	(1)
Share-based compensation	16		17	(1)
Ending balance at Dec. 31, 2018	$ 12,222	$ 1,285	$ 6,168	$ 4,893	$ (124)
Balance (in shares) at Dec. 31, 2018		514,037
Increase (Decrease) in Stockholders' Equity
Dividends declared on common stock	$ (1.52)