Stockholders' Equity - Stock Repurchases and Recent Sales of Unregistered Securities (Details) - USD ($) $ / shares in Units, $ in Millions		2 Months Ended		3 Months Ended	10 Months Ended	12 Months Ended		14 Months Ended	27 Months Ended
	Jul. 01, 2016	Feb. 23, 2018	Dec. 31, 2014	Dec. 31, 2015	Oct. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Oct. 26, 2015	Oct. 22, 2014
AFSC
Recent Sales of Unregistered Securities
Number of restricted stock purchased by certain principal owners of the acquiree	60,069
Restricted common stock issued	$ 4.0
Trading day period to calculate average of the closing prices of the company's stock	5 days
Vesting period of restricted stock issued	2 years
Vesting percentage on the first anniversary of the acquisition	50.00%
Vesting percentage on the second anniversary of the acquisition	50.00%
October 2014 Share Repurchase Program
Stock Repurchases
Amount authorized under stock repurchase plan											$ 200.0
Total number of Shares Purchased			232,170		3,153,156			3,385,326
Average Price Paid per Share (in dollars per share)			$ 60.65		$ 58.96
Aggregate Cost			$ 14.1		$ 185.9			$ 200.0
October 2015 Share Repurchase Program
Stock Repurchases
Amount authorized under stock repurchase plan										$ 200.0
Total number of Shares Purchased		0		345,044		280,140	1,828,183		2,453,367
Average Price Paid per Share (in dollars per share)				$ 53.46		$ 77.67	$ 58.40
Aggregate Cost				$ 18.4		$ 21.8	$ 106.8		$ 147.0