Net Income per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 42,981	$ 46,161	$ 92,005	$ 88,533
Net Income allocated to participating securities	(383)	(684)	(879)	(1,268)
Net Income Allocated to Common Stockholders	$ 42,598	$ 45,477	$ 91,126	$ 87,265
Weighted Average Number of Shares Outstanding, Basic	85,831,000	87,341,000	86,140,000	87,307,000
Weighted Average Number of Shares Outstanding, Diluted (in shares)	85,831,000	87,341,000	86,140,000	87,307,000
Weighted Average Number Diluted Shares Outstanding Adjustment	0	0	0	0
Basic (in dollars per share)	$ 0.50	$ 0.52	$ 1.06	$ 1.00
Diluted (in dollars per share)	$ 0.50	$ 0.52	$ 1.06	$ 1.00