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, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Earnings Per Share Basic and Diluted By Common Class Including Two Class Method [Line Items]
Net income available for stockholders:	$ 166,895	$ 274,436	$ 296,381
Less: Dividends paid
Undistributed earnings	79,106	160,367	249,054
Basic shares outstanding:
Basic shares outstanding (in shares)	215,504	215,241	217,683
Diluted shares outstanding:
Basic shares outstanding (in shares)	215,504	215,241	217,683
Diluted shares outstanding (in shares)	216,733	216,796	220,250
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ 0.77	$ 1.28	$ 1.36
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ 0.77	$ 1.27	$ 1.35
Common Stock
Less: Dividends paid
Dividends	(86,282)	(111,966)	(46,479)
Undistributed earnings	77,620	157,093	244,053
Basic shares outstanding:
Basic shares outstanding (in shares)	211,305	210,707	213,153
Diluted shares outstanding:
Basic shares outstanding (in shares)	211,305	210,707	213,153
Dilutive effect of stock-based awards (in shares)	1,229	1,555	2,567
Diluted shares outstanding (in shares)	212,534	212,262	215,720
Basic earnings per share:
Distributed earnings (in dollars per share)	$ 0.40	$ 0.53	$ 0.22
Undistributed earnings (in dollars per share)	$ 0.37	$ 0.75	$ 1.14
Basic earnings per share (in dollars per share)	$ 0.77	$ 1.28	$ 1.36
Diluted earnings per share:
Distributed earnings (in dollars per share)	$ 0.40	$ 0.53	$ 0.22
Undistributed earnings (in dollars per share)	$ 0.37	$ 0.74	$ 1.13
Diluted earnings per share (in dollars per share)	$ 0.77	$ 1.27	$ 1.35
Restricted shares of common stock
Less: Dividends paid
Dividends	(1,507)	(2,103)	(848)
Undistributed earnings	$ 1,486	$ 3,274	$ 5,001
Basic shares outstanding:
Basic shares outstanding (in shares)	4,199	4,534	4,530
Diluted shares outstanding:
Basic shares outstanding (in shares)	4,199	4,534	4,530
Diluted shares outstanding (in shares)	4,199	4,534	4,530
Basic earnings per share:
Distributed earnings (in dollars per share)	$ 0.36	$ 0.46	$ 0.19
Undistributed earnings (in dollars per share)	$ 0.35	$ 0.72	$ 1.10
Basic earnings per share (in dollars per share)	$ 0.71	$ 1.18	$ 1.29