EPS Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income		$ 73,471	$ 79,510	$ 78,071	$ 82,174	$ 79,708	$ 78,882	$ 78,726	$ 76,188	$ 313,226	$ 313,504	$ 302,709
Dividends declared on preferred stock										(15,000)	(15,000)	(13,750)
Dividends, dividend equivalents and undistributed earnings allocated to participating securities										(1,539)	(1,106)	(1,080)
Issuance costs of redeemed preferred stock	[1]									0	0	(3,947)
Net income available to common stockholders - basic and diluted										$ 296,687	$ 297,398	$ 283,932
Denominator:
Weighted average common shares for basic and diluted earnings per share	[2]									372,114	369,048	362,399
Basic and diluted earnings per share (in dollars per share)		$ 0.19	$ 0.20	$ 0.20	$ 0.21	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.80	$ 0.81	$ 0.78
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)										2,000
Restricted common stock
Denominator:
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)										111
Weighted average grant date fair value (in dollars per share)		7.41								$ 7.41
Restricted Stock Units
Denominator:
Anti-dilutive securities excluded from diluted earnings per share calculations (in shares)										1,900
Weighted average grant date fair value (in dollars per share)		$ 6.90								$ 6.90

[1] Issuance costs of redeemed preferred stock represent the original offering costs related to the Series A Preferred Stock, which was redeemed on May 16, 2013. (See Note 13)

[2] At December 31, 2015, the Company had an aggregate of 2.0 million equity instruments outstanding that were not included in the calculation of diluted EPS for the year ended December 31, 2015, as their inclusion would have been anti-dilutive. These equity instruments were comprised of approximately 111,000 shares of restricted common stock with a weighted average grant date fair value of $7.41 and approximately $1.9 million RSUs with a weighted average grant date fair value of $6.90. These equity instruments may have a dilutive impact on future EPS.