Subsequent Event (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	0 Months Ended	6 Months Ended					3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	0 Months Ended	1 Months Ended
	Aug. 01, 2013	Jun. 30, 2013	Jun. 30, 2013 Swaps		Dec. 31, 2012 Swaps		Jun. 30, 2013 Swaps Low end of range	Jun. 30, 2013 Swaps Low end of range	Jun. 30, 2013 Swaps High end of range	Jun. 30, 2013 Swaps High end of range	Aug. 01, 2013 Subsequent event	Jul. 31, 2013 Subsequent event Swaps derivative	Jul. 31, 2013 Subsequent event Swaps Derivative Maturity 5 Years	Jul. 31, 2013 Subsequent event Swaps Derivative Maturity 7 Years	Jul. 31, 2013 Subsequent event Swaps Derivative Maturity 10 years	Jul. 31, 2013 Subsequent event Swaps Low end of range	Jul. 31, 2013 Subsequent event Swaps High end of range
Subsequent event
Dividends Declared per Share of Common Stock, special dividend (in dollars per share)	$ 0.28	$ 0.50									$ 0.28
Number of new derivatives												8
Notional Amount of Derivatives Entered												$ 950,000	$ 350,000	$ 500,000	$ 100,000
Fixed-pay rate (as a percent)												1.97%	1.00%	2.14%	2.75%
Initial Term of Derivative Entered							2 months	2 months	7 years	7 years						5 years	10 years
Aggregate notional amount of derivatives			$ 2,687,275	[1]	$ 2,519,564	[1]						$ 3,452,000
Weighted Average Fixed-Pay Interest Rate (as a percent)			2.10%	[1]	2.31%	[1]						1.96%
Average maturity term of swaps			27 months									43 months

[1] Each maturity category reflects contractual amortization and/or maturity of notional amounts.