CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (UNAUDITED) (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock [Member]	Additional Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Total
Balance at Dec. 31, 2010	$ 1,205,834	$ 5,229,075	$ 1,701,703	$ (53,093)	$ 8,083,519
Balance (in shares) at Dec. 31, 2010	482,334,000
Comprehensive income:
Net income			203,569		203,569
Other comprehensive income				1,246	1,246
Comprehensive income					204,815
Dividends declared:
Cumulative preferred stock			(1,060)		(1,060)
Common stock			(122,826)		(122,826)
Issuances of common stock	4,577	1,652			6,229
Issuances of common stock (in shares)	1,831,000
Share-based compensation		10,806			10,806
Balance at Mar. 31, 2011	1,210,411	5,241,533	1,781,386	(51,847)	8,181,483
Balance (in shares) at Mar. 31, 2011	484,165,000
Balance at Dec. 31, 2011	1,216,234	5,327,443	2,032,556	(94,035)	8,482,198
Balance (in shares) at Dec. 31, 2011	486,494,000				486,493,933
Comprehensive income:
Net income			183,893		183,893
Other comprehensive income				26,520	26,520
Comprehensive income					210,413
Dividends declared:
Common stock			(127,174)		(127,174)
Issuances of common stock	2,855	2,288			5,143
Issuances of common stock (in shares)	1,142,000
Repurchase of common stock	(1,750)	(16,779)			(18,529)
Repurchase of common stock (in shares)	(700,000)
Purchase of common stock for settlement of equity awards		(23,307)			(23,307)
Share-based compensation		8,927			8,927
Balance at Mar. 31, 2012	$ 1,217,339	$ 5,298,572	$ 2,089,275	$ (67,515)	$ 8,537,671
Balance (in shares) at Mar. 31, 2012	486,936,000				486,935,997