Stock Incentive Plans (Details) - USD ($) $ / shares in Units, $ in Millions	4 Months Ended	8 Months Ended	12 Months Ended
	May 08, 2018	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock incentive plan
Minimum exercise price to fair market value on the date of grant (as a percent)			100.00%
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	5 years	10 years
Share-based Compensation Arrangement by Share-based Payment Award, Plan Modification, Incremental Compensation Cost					$ 0.6
Stock Option Activity Roll Forward, Number of Shares
Options exercised (in shares)			(243,502)	(386,300)	(745,938)
Employee nonstatutory stock options
Stock incentive plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value			$ 12.30	$ 13.00	$ 12.90
Stock Option Activity Roll Forward, Number of Shares
Options outstanding at the beginning of the period (in shares)	1,813,450		1,813,450	1,940,785	2,320,687
Options exercised (in shares)			(243,502)	(386,300)	(745,938)
Options forfeited (in shares)			(90,862)	(34,545)	(9,458)
Options terminated (in shares)			(33,481)	(23,320)	(2,426)
Options granted (in shares)			428,190	316,830	377,920
Options outstanding at the end of the period (in shares)		1,873,795	1,873,795	1,813,450	1,940,785
Stock Option Activity Roll Forward, Average Exercise Price
Average exercise price at the beginning of the period (in dollars per share)	$ 72.04		$ 72.04	$ 66.83	$ 71.01
Options exercised (in dollars per share)			61.92	45.21	36.97
Options forfeited (in dollars per share)			69.82	72.97	73.40
Options terminated (in dollars per share)			75.07	75.60	67.06
Options granted (in dollars per share)			67.36	71.37	68.32
Average exercise price at the end of the period (in dollars per share)		$ 72.34	$ 72.34	$ 72.04	$ 66.83
Stock Option Activity Roll Forward, Non-vested, Number of Shares
Nonvested options outstanding at the beginning of the period (in shares)		664,363	664,363	719,027
Options granted (in shares)			428,190	316,830	377,920
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares			(358,511)
Forfeited (in shares)			(124,343)
Stock Option Activity Roll Forward, Non-vested, Weighted-Average Grant Date Fair Value
Share-based Compensation Arrangement by Share-based Payment Award, Option, Nonvested, Weighted Average Exercise Price		$ 12.88	$ 12.88	$ 15.47
Vested/Exercised (in dollars per share)			18.14
Forfeited (in dollars per share)			$ 14.99
Summary of intrinsic value
Total intrinsic value of all options exercised			$ 1.6	$ 10.3	$ 25.9
Minimum
Stock incentive plan
Stock incentive plan vesting period			1 year
Maximum
Stock incentive plan
Stock incentive plan vesting period			3 years