Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 06, 2019	Jul. 27, 2015
Class A
Subsequent events
Value of shares of the entity's Class A common stock authorized to be repurchased		$ 100
Subsequent event | Class A
Subsequent events
Quarterly dividend payable (in dollars per share)	$ 0.19
Subsequent event | Class A | Maximum
Subsequent events
Value of shares of the entity's Class A common stock authorized to be repurchased	$ 150
Subsequent event | Class B
Subsequent events
Quarterly dividend payable (in dollars per share)	$ 0.21