Goodwil (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Impairment loss as a result of goodwill	$ 0	$ 0	$ 0
Goodwill [Roll Forward]
Balance at the beginning of the period	431,700,000	431,400,000
Foreign currency translation	(300,000)	300,000
Goodwill acquired	154,100,000
Balance at the end of the period	585,500,000	431,700,000	431,400,000
Segment One
Goodwill [Roll Forward]
Balance at the beginning of the period	369,000,000	368,700,000
Foreign currency translation	(300,000)	300,000
Goodwill acquired	0
Balance at the end of the period	368,700,000	369,000,000	368,700,000
Segment Two
Goodwill [Roll Forward]
Balance at the beginning of the period	62,700,000	62,700,000
Foreign currency translation	0	0
Goodwill acquired	154,100,000
Balance at the end of the period	$ 216,800,000	$ 62,700,000	$ 62,700,000