Retirement Plans - Defined Contribution Plans (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Retirement Benefits [Abstract]
Maximum percentage of employee compensation match by employer to employee stock ownership plan (percent)			4.00%
Employer contributions	$ 3.3	$ 3.0	$ 11.5	$ 10.2
Maximum percent of employer matching contribution in form of stock (percent)			50.00%
Shares held in defined contribution plan (shares)	1.2		1.2		1.2