Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 306.2	$ 592.7	$ 633.7
Available-for-sale securities:
Unrealized gains net of tax provision of $4.0, tax benefit of $0.7, and tax provision of $6.5 for 2017, 2016, and 2015, respectively	4.5	0.8	9.1
Reclassification adjustments for realized net gains included in net income, net of tax provisions of $0.9 and $0.5, and zero for 2017, 2016, and 2015, respectively	(2.1)	(1.2)	(0.5)
Net change on available-for-sale securities, net of tax	2.4	(0.4)	8.6
Cash flow hedges:
Unrealized gain (losses) net of tax provisions of $4.4, $0.8, and $0.4 for 2017, 2016, and 2015, respectively	15.7	(2.1)	(6.7)
Reclassification adjustments for realized (gains) loss included in net income, net of tax provisions of $2.4 and $0.7, and zero for 2017, 2016, and 2015, respectively	(5.2)	(1.1)	9.6
Net change on cash flow hedges, net of tax	10.5	(3.2)	2.9
Change in foreign currency translation adjustments	19.0	(14.5)	(16.9)
Other comprehensive income (loss), net of tax	31.9	(18.1)	(5.4)
Comprehensive income	$ 338.1	$ 574.6	$ 628.3