CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2015	$ 10,600,920	$ 1,268,839	$ 5,889,106	$ 3,552,728	$ (109,753)
Balance (in shares) at Dec. 31, 2015		507,536,000
Increase (Decrease) in Stockholders' Equity
Net income	241,312			241,312
Other comprehensive income	1,145				1,145
Dividends declared on common stock	(173,619)			(173,619)
Issuances of common stock (in shares)		417,000
Issuances of common stock	(2,712)	$ (1,043)	(3,755)
Purchase of common stock for settlement of equity awards	(789)		789
Share-based compensation	5,377		5,377
Ending balance at Mar. 31, 2016	10,671,634	$ 1,269,882	5,889,939	3,620,421	(108,608)
Balance (in shares) at Mar. 31, 2016		507,953,000
Beginning balance at Dec. 31, 2016	$ 11,020,849	$ 1,268,057	5,881,494	3,981,652	(110,354)
Balance (in shares) at Dec. 31, 2016	507,222,795	507,223,000
Increase (Decrease) in Stockholders' Equity
Net income	$ 239,277			239,277
Other comprehensive income	1,773				1,773
Dividends declared on common stock	(183,815)			(183,815)
Issuances of common stock (in shares)		611,000
Issuances of common stock	(5,037)	$ (1,527)	(3,510)
Purchase of common stock for settlement of equity awards	(2,943)
Purchase of common stock (in shares)		(71,000)
Repurchases of common stock	(3,120)	$ (177)	(2,943)
Share-based compensation	(9,890)		(9,128)	(762)
Ending balance at Mar. 31, 2017	$ 11,070,111	$ 1,269,407	$ 5,872,933	$ 4,036,352	$ (108,581)
Balance (in shares) at Mar. 31, 2017	507,762,881	507,763,000