Significant Accounting Policies - Summary of reconciliation of weighted average shares outstanding along with earnings (loss) per share attributable to restricted shares of common stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Earnings Per Share Basic and Diluted By Common Class Including Two Class Method [Line Items]
Net income available for stockholders:	$ 274,436	$ 296,381	$ 146,742
Less: Dividends paid
Undistributed earnings	160,367	249,054	126,095
Basic shares outstanding :
Basic shares outstanding (in shares)	215,241	217,683	217,484
Diluted shares outstanding :
Basic shares outstanding (in shares)	215,241	217,683	217,484
Diluted shares outstanding (in shares)	216,796	220,250	219,805
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ 1.28	$ 1.36	$ 0.67
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ 1.27	$ 1.35	$ 0.67
Common Stock
Less: Dividends paid
Dividends	(111,966)	(46,479)	(20,294)
Undistributed earnings	157,093	244,053	123,536
Basic shares outstanding :
Basic shares outstanding (in shares)	210,707	213,153	212,799
Diluted shares outstanding :
Basic shares outstanding (in shares)	210,707	213,153	212,799
Dilutive effect of stock-based awards (in shares)	1,555	2,567	2,321
Diluted shares outstanding (in shares)	212,262	215,720	215,120
Basic earnings per share:
Distributed earnings (in dollars per share)	$ 0.53	$ 0.22	$ 0.10
Undistributed earnings (in dollars per share)	$ 0.75	$ 1.14	$ 0.57
Basic earnings per share (in dollars per share)	$ 1.28	$ 1.36	$ 0.67
Diluted earnings per share:
Distributed earnings (in dollars per share)	$ 0.53	$ 0.22	$ 0.10
Undistributed earnings (in dollars per share)	$ 0.74	$ 1.13	$ 0.57
Diluted earnings per share (in dollars per share)	$ 1.27	$ 1.35	$ 0.67
Restricted shares of common stock
Less: Dividends paid
Dividends	(2,103)	(848)	(353)
Undistributed earnings	$ 3,274	$ 5,001	$ 2,559
Basic shares outstanding :
Basic shares outstanding (in shares)	4,534	4,530	4,685
Diluted shares outstanding :
Basic shares outstanding (in shares)	4,534	4,530	4,685
Diluted shares outstanding (in shares)	4,534	4,530	4,685
Basic earnings per share:
Distributed earnings (in dollars per share)	$ 0.46	$ 0.19	$ 0.08
Undistributed earnings (in dollars per share)	$ 0.72	$ 1.10	$ 0.55
Basic earnings per share (in dollars per share)	$ 1.18	$ 1.29	$ 0.63