Income Taxes (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Income Taxes
Income tax expense	$ 34,404,000	$ 27,052,000	$ 90,521,000	$ 73,735,000
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	$ 400,000