Earnings Per Share - Calculation of EPS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net income	$ 24,464	$ 43,808	$ 59,928	$ 99,762
Denominator:
Weighted-average number of common shares outstanding—basic (in shares)	200,852,072	195,796,772	200,776,641	195,733,185
Weighted-average number of common shares outstanding—diluted (in shares)	201,167,659	196,434,773	201,124,091	196,341,317
Earnings per share:
Basic earnings per share (in dollars per share)	$ 0.12	$ 0.22	$ 0.30	$ 0.51
Diluted earnings per share (in dollars per share)	$ 0.12	$ 0.22	$ 0.30	$ 0.51
Stock Appreciation Rights (SARs) [Member]
Earnings Per Share [Line Items]
Antidilutive securities excluded from calculation (in shares)	20,770			262,461
Unvested restricted common stock [Member]
Denominator:
Unvested Stock/SARs/MSUs (in shares)	99,873	215,834	130,349	193,477
Stock Appreciation Rights [Member]
Denominator:
Unvested Stock/SARs/MSUs (in shares)	0	7,512	1,387	0
Shares related to unvested PSUs [Member]
Denominator:
Unvested Stock/SARs/MSUs (in shares)	215,714	414,655	215,714	414,655