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			362,331		362,331
Other comprehensive income				2,110	2,110
Comprehensive income					364,441
Dividends declared:
Cumulative preferred stock			(2,120)		(2,120)
Common stock			(249,409)		(249,409)
Issuances of common stock	5,522	9,724			15,246
Issuances of common stock (in shares)	2,209,000
Share-based compensation		22,888			22,888
Balance at Jun. 30, 2011	1,211,356	5,261,687	1,812,505	(50,983)	8,234,565
Balance (in shares) at Jun. 30, 2011	484,543,000
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
Comprehensive income:
Net income			158,762		158,762
Other comprehensive income				864	864
Comprehensive income					159,626
Dividends declared:
Cumulative preferred stock			(1,060)		(1,060)
Common stock			(126,583)		(126,583)
Issuances of common stock	945	8,072			9,017
Issuances of common stock (in shares)	378,000
Share-based compensation		12,082			12,082
Balance at Jun. 30, 2011	1,211,356	5,261,687	1,812,505	(50,983)	8,234,565
Balance (in shares) at Jun. 30, 2011	484,543,000
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			366,953		366,953
Other comprehensive income				(7,971)	(7,971)
Comprehensive income					358,982
Dividends declared:
Common stock			(258,870)		(258,870)
Issuances of common stock	3,730	10,770			14,500
Issuances of common stock (in shares)	1,492,000
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		18,531			18,531
Balance at Jun. 30, 2012	1,218,214	5,316,658	2,140,639	(102,006)	8,573,505
Balance (in shares) at Jun. 30, 2012	487,286,000				487,285,506
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
Comprehensive income:
Net income			183,060		183,060
Other comprehensive income				(34,491)	(34,491)
Comprehensive income					148,569
Dividends declared:
Common stock			(131,696)		(131,696)
Issuances of common stock	875	8,482			9,357
Issuances of common stock (in shares)	350,000
Share-based compensation		9,604			9,604
Balance at Jun. 30, 2012	$ 1,218,214	$ 5,316,658	$ 2,140,639	$ (102,006)	$ 8,573,505
Balance (in shares) at Jun. 30, 2012	487,286,000				487,285,506