Summary of Significant Accounting Policies (Effects of Adoption of Topic 606 - Unaudited Consolidated Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 615.5	$ 240.3	$ 1,166.1	$ 615.3
Adjustments to reconcile net income to net cash provided by operating activities:
Increase in prepayments and other assets			61.5	(104.6)
Decrease in accounts payable and other liabilities			(186.3)	$ (445.1)
Before Adoption of Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 642.5		1,214.9
Adjustments to reconcile net income to net cash provided by operating activities:
Increase in prepayments and other assets			(5.5)
Decrease in accounts payable and other liabilities			$ (183.2)