Accumulated Other Comprehensive Income/(Loss) (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	$ 69.4	$ 9.3
Unrealized holding gain/(loss) on available-for-sale debt securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	$ 67.9	$ 4.4
ASU 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effect adjustment			$ 67.9
ASU 2016-01 | Retained earnings
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative-effect adjustment			$ 67.9