Summary of Significant Accounting Policies (Effects of Adoption of Topic 606 - Unaudited Consolidated Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 537.2	$ 550.8	$ 1,703.3	$ 1,166.1
Adjustments to reconcile net income to net cash provided by operating activities:
Decrease in prepayments and other assets			44.6	70.4
Decrease in accounts payable and other liabilities			(244.8)	$ (557.8)
Before Adoption of Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 535.5		1,717.4
Adjustments to reconcile net income to net cash provided by operating activities:
Decrease in prepayments and other assets			100.2
Decrease in accounts payable and other liabilities			$ (241.8)