Stockholders' Equity (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended	16 Months Ended
Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Jun. 30, 2017
Stockholders' Equity
Cost of shares of Class A common stock repurchased	$ 8,599	$ 4,826	$ 8,599
Class A Common Stock
Stockholders' Equity
Authorized stock repurchase amount					$ 50,000
Shares of Class A common stock repurchased	505	236	505	741
Cost of shares of Class A common stock repurchased	$ 8,599	$ 4,826	$ 8,599	$ 13,425

Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration