Retirement Plans (401(k) Savings Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Employer discretionary contribution percent	4.00%
Maximum annual contribution per employee percent	50.00%
Employer matching contribution percent	4.00%
Employer matching contribution amount	$ 2.9	$ 3.0	$ 1.6
Contribution to 401(k) savings plan	6.2	3.5	3.1
Employer total contribution amount	$ 9.1	$ 6.5	$ 4.7