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Subsequent Events (Details) - Class A common stock - USD ($)		1 Months Ended	9 Months Ended	32 Months Ended
	Oct. 22, 2018	Oct. 26, 2018	Sep. 30, 2018	Sep. 30, 2018	Feb. 29, 2016
Subsequent Event [Line Items]
Shares repurchased pursuant to repurchase authorizations (in shares)			1,199,257	2,241,982
Authorized repurchase amount					$ 250,000,000
2016 Share Repurchase Authorization
Subsequent Event [Line Items]
Remaining authorized repurchase amount			$ 50,900,000	$ 50,900,000
Subsequent Event
Subsequent Event [Line Items]
Shares repurchased pursuant to repurchase authorizations (in shares)		423,863
Average purchase price per share (in dollars per share)		$ 76.85
Increase in authorized repurchase amount	$ 250,000,000
Subsequent Event | 2016 Share Repurchase Authorization
Subsequent Event [Line Items]
Remaining authorized repurchase amount		$ 18,300,000
Subsequent Event | 2018 Stock Repurchase Authorization
Subsequent Event [Line Items]
Authorized repurchase amount		$ 268,000,000

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Stock Repurchase Program, Increase (Decrease) In Authorized Repurchase Amount

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Total cost of shares repurchased divided by the total number of shares repurchased.

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