Commitments and Contingencies (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	2 Months Ended	6 Months Ended	12 Months Ended		33 Months Ended
	May 24, 2018	Jul. 20, 2018	Dec. 31, 2015	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Oct. 26, 2015
Stock Repurchases
Percentage of total net revenues generated by AFSC				2.00%	3.00%
October 2015 Share Repurchase Program
Stock Repurchases
Amount authorized under stock repurchase plan	$ 400.0							$ 200.0
Increase in amount authorized under stock repurchase plan	$ 200.0
Share repurchases made in open market (in shares)		37,032	345,044	163,532	280,140	1,828,183	2,616,899
Average Price Paid per Share (in dollars per share)			$ 53.46	$ 90.62	$ 77.67	$ 58.40
Aggregate Cost		$ 3.6	$ 18.4	$ 14.8	$ 21.8	$ 106.8	$ 161.8
Remaining authorized repurchase amount				$ 238.2			$ 238.2