EPS Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income	$ 78,071	$ 78,726	$ 160,244	$ 154,914
Dividends declared on preferred stock	(3,750)	(3,750)	(7,500)	(7,500)
Dividends, dividend equivalents and undistributed earnings allocated to participating securities	(391)	(287)	(786)	(561)
Net income to common stockholders - basic and diluted	$ 73,930	$ 74,689	$ 151,958	$ 146,853
Denominator:
Weighted average common shares for basic and diluted earnings per share (in shares)	370,164	368,445	371,992	367,792
Basic and diluted earnings per share (in dollars per share)	$ 0.20	$ 0.20	$ 0.41	$ 0.40
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	2,100		2,100
Restricted Common Stock
Denominator:
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	203		203
Weighted average grant date fair value (in dollars per share)	$ 7.49		$ 7.49
Restricted Stock Units
Denominator:
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)	1,900		1,900
Weighted average grant date fair value (in dollars per share)	$ 6.89		$ 6.89