Significant Accounting Policies - Summary of reconciliation of weighted average shares outstanding along with earnings per share attributable to restricted shares of common stock (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Earnings Per Share Basic and Diluted By Common Class Including Two Class Method [Line Items]
Net income available for stockholders:	$ 245,193	$ 166,895	$ 274,436
Less: Dividends paid
Undistributed earnings	153,585	79,106	160,367
Basic shares outstanding:
Basic shares outstanding (in shares)	214,840	215,504	215,241
Diluted shares outstanding:
Basic shares outstanding (in shares)	214,840	215,504	215,241
Diluted shares outstanding (in shares)	215,889	216,733	216,796
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ 1.14	$ 0.77	$ 1.28
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ 1.14	$ 0.77	$ 1.27
Common Stock
Less: Dividends paid
Dividends	(90,231)	(86,282)	(111,966)
Undistributed earnings	151,049	77,620	157,093
Basic shares outstanding:
Basic shares outstanding (in shares)	211,208	211,305	210,707
Diluted shares outstanding:
Basic shares outstanding (in shares)	211,208	211,305	210,707
Dilutive effect of stock-based awards (in shares)	1,049	1,229	1,555
Diluted shares outstanding (in shares)	212,257	212,534	212,262
Basic earnings per share:
Distributed earnings (in dollars per share)	$ 0.43	$ 0.40	$ 0.53
Undistributed earnings (in dollars per share)	$ 0.71	$ 0.37	$ 0.75
Basic earnings per share (in dollars per share)	$ 1.14	$ 0.77	$ 1.28
Diluted earnings per share:
Distributed earnings (in dollars per share)	$ 0.43	$ 0.40	$ 0.53
Undistributed earnings (in dollars per share)	$ 0.71	$ 0.37	$ 0.74
Diluted earnings per share (in dollars per share)	$ 1.14	$ 0.77	$ 1.27
Restricted shares of common stock
Less: Dividends paid
Dividends	(1,377)	(1,507)	(2,103)
Undistributed earnings	$ 2,536	$ 1,486	$ 3,274
Basic shares outstanding:
Basic shares outstanding (in shares)	3,632	4,199	4,534
Diluted shares outstanding:
Basic shares outstanding (in shares)	3,632	4,199	4,534
Diluted shares outstanding (in shares)	3,632	4,199	4,534
Basic earnings per share:
Distributed earnings (in dollars per share)	$ 0.38	$ 0.36	$ 0.46
Undistributed earnings (in dollars per share)	$ 0.70	$ 0.35	$ 0.72
Basic earnings per share (in dollars per share)	$ 1.08	$ 0.71	$ 1.18