Net Income per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 46,161	$ 38,496	$ 88,533	$ 71,913
Net Income allocated to participating securities	(684)	(593)	(1,268)	(1,146)
Net Income Allocated to Common Stockholders	$ 45,477	$ 37,903	$ 87,265	$ 70,767
Weighted Average Number of Shares Outstanding, Basic (in shares)	87,341,000	87,153,000	87,307,000	87,649,000
Weighted Average Number of Shares Outstanding, Diluted (in shares)	87,341,000	87,153,000	87,307,000	87,649,000
Weighted Average Number Diluted Shares Outstanding Adjustment	0	0	0	0
Basic (in dollars per share)	$ 0.52	$ 0.44	$ 1.00	$ 0.81
Diluted (in dollars per share)	$ 0.52	$ 0.44	$ 1.00	$ 0.81