Income Taxes (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Income Taxes
Income tax expense	$ 29,829,000	$ 25,422,000	$ 56,117,000	$ 46,683,000
Accrued interest and penalties relating to uncertain tax positions	2,700,000		2,700,000		3,000,000
Interest and penalties expense or benefit relating to uncertain tax positions			$ 300,000	$ 500,000