Postretirement Benefits (Details 12) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee savings plan
Employer matching contribution paid to the plan	$ 12.1	$ 11.6	$ 10.7
Class A common stock held by plan participants (in shares)	0.2	0.2
100% employer matching contribution
Employee savings plan
Percentage of employer matching contribution of employee contributions (as a percent)	100.00%
100% employer matching contribution | Maximum
Employee savings plan
Percentage of eligible employee compensation, matched by employer (as a percent)	3.00%
50% employer matching contribution
Employee savings plan
Percentage of employer matching contribution of employee contributions (as a percent)	50.00%
50% employer matching contribution | Maximum
Employee savings plan
Percentage of eligible employee compensation, matched by employer (as a percent)	5.00%
50% employer matching contribution | Minimum
Employee savings plan
Percentage of eligible employee compensation, matched by employer (as a percent)	3.00%