Employee Benefit Plans - Defined Contribution Retirement Plans (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
The DC Plan
Defined Contribution Retirement Plans
Maximum voluntary contribution by eligible U.S. employees as a percentage of their compensation	50.00%
Number of shares contributed	334	605	434
Compensation expense	$ 8.7	$ 8.8	$ 10.2
Supplemental Retirement Savings Plan (SRSP)
Defined Contribution Retirement Plans
Compensation expense	0.2	0.7	$ 0.7
Supplemental Retirement Savings Plan (SRSP) | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Contribution Retirement Plans
Fair value of assets	$ 21.2	$ 19.1