Summary of Significant Accounting Policies (Effects of Adoption of Topic 606 - Unaudited Consolidated Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 550.6	$ 375.0
Adjustments to reconcile net income to net cash provided by operating activities:
Increase in prepayments and other assets	136.7	(14.2)
Decrease in accounts payable and other liabilities	(172.6)	$ (671.5)
Before Adoption of Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	572.4
Adjustments to reconcile net income to net cash provided by operating activities:
Increase in prepayments and other assets	(92.0)
Decrease in accounts payable and other liabilities	(159.4)
Effect of change | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	(21.8)
Adjustments to reconcile net income to net cash provided by operating activities:
Increase in prepayments and other assets	(44.7)
Decrease in accounts payable and other liabilities	$ (13.2)