Debt (Senior Credit Agreement) (Narrative) (Details) - USD ($)				1 Months Ended	3 Months Ended
	May 07, 2018	Apr. 11, 2018	Oct. 13, 2017	Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017
Line of Credit Facility [Line Items]
Repayments of revolving credit facility					$ 70,000,000	$ 110,000,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity					$ 1,000,000,000
Revolving credit facility, effective interest rate					3.74%
Line of credit facility, commitment fee					0.25%
Borrowings on the revolving credit facility					$ 265,000,000
Repayments of revolving credit facility			$ 460,000,000
Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Borrowings on the revolving credit facility				$ 200,000,000
Repayments of revolving credit facility				$ 335,000,000
Line of credit facility, outstanding	$ 100,000,000
Amended Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 1,250,000,000
Revolving credit facility, maturity date		Apr. 11, 2023
Revolving credit facility, effective interest rate	3.26%
Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate					1.375%
Minimum [Member] | Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate	1.125%
Minimum [Member] | Revolving Credit Facility [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate					0.375%
Minimum [Member] | Amended Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment fee		0.20%
Minimum [Member] | Amended Revolving Credit Facility [Member] | Eurodollar [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate		1.125%
Minimum [Member] | Amended Revolving Credit Facility [Member] | Base Rate [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate		0.125%
Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate					2.00%
Maximum [Member] | Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate	1.75%
Maximum [Member] | Revolving Credit Facility [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate					1.00%
Maximum [Member] | Amended Revolving Credit Facility [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment fee		0.25%
Maximum [Member] | Amended Revolving Credit Facility [Member] | Eurodollar [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate		1.75%
Maximum [Member] | Amended Revolving Credit Facility [Member] | Base Rate [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate		0.75%
2018 Term Loan B [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount		$ 2,400,000,000
2018 Term Loan B [Member] | Eurodollar [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate		2.00%
2018 Term Loan B [Member] | Base Rate [Member] | Subsequent Event [Member]
Line of Credit Facility [Line Items]
Basis spread on variable interest rate		1.00%