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 ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Earnings Per Share Basic and Diluted By Common Class Including Two Class Method [Line Items]
Net (loss) income available for stockholders:	$ (99,561)	$ 245,193	$ 166,895
Less: Dividends paid
(Over distributed loss) Undistributed earnings	$ (133,163)	$ 153,585	$ 79,106
Basic shares outstanding:
Basic shares outstanding (in shares)	213,632	214,840	215,504
Diluted shares outstanding:
Basic shares outstanding (in shares)	213,632	214,840	215,504
Basic (loss) earnings per share:
Basic (loss) earnings per share (in dollars per share)	$ (0.47)	$ 1.14	$ 0.77
Diluted (loss) earnings per share:
Diluted (loss) earnings per share (in dollars per share)	$ (0.47)	$ 1.14	$ 0.77
Common Stock
Less: Dividends paid
Dividends	$ (33,120)	$ (90,231)	$ (86,282)
(Over distributed loss) Undistributed earnings	$ (131,130)	$ 151,049	$ 77,620
Basic shares outstanding:
Basic shares outstanding (in shares)	210,273	211,208	211,305
Diluted shares outstanding:
Basic shares outstanding (in shares)	210,273	211,208	211,305
Dilutive effect of stock-based awards (in shares)		1,049	1,229
Diluted shares outstanding (in shares)	210,273	212,257	212,534
Basic (loss) earnings per share:
Distributed (loss) earnings (in dollars per share)	$ 0.16	$ 0.43	$ 0.40
(Over distributed loss) Undistributed earnings (in dollars per share)	(0.63)	0.71	0.37
Basic (loss) earnings per share (in dollars per share)	(0.47)	1.14	0.77
Diluted (loss) earnings per share:
Distributed earnings (in dollars per share)	0.16	0.43	0.40
Undistributed earnings (in dollars per share)	(0.63)	0.71	0.37
Diluted (loss) earnings per share (in dollars per share)	$ (0.47)	$ 1.14	$ 0.77
Restricted shares of common stock
Less: Dividends paid
Dividends	$ (482)	$ (1,377)	$ (1,507)
(Over distributed loss) Undistributed earnings	$ (2,033)	$ 2,536	$ 1,486
Basic shares outstanding:
Basic shares outstanding (in shares)	3,359	3,632	4,199
Diluted shares outstanding:
Basic shares outstanding (in shares)	3,359	3,632	4,199
Basic (loss) earnings per share:
Distributed (loss) earnings (in dollars per share)	$ 0.14	$ 0.38	$ 0.36
(Over distributed loss) Undistributed earnings (in dollars per share)	(0.61)	0.70	0.35
Basic (loss) earnings per share (in dollars per share)	$ (0.47)	$ 1.08	$ 0.71