Debt Debt (Schedule of Revolving Credit Facility) (Details) - Senior Unsecured Revolving Credit Facility [Member] - Revolving Credit Facility [Member] $ in Millions	Mar. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Borrowings outstanding	$ 0
Letters of credit issued	0
Available for borrowing	$ 1,000	[1]
London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.50%

[1] The margin for borrowings under the senior unsecured revolving credit facility was 1.5% above LIBOR at current Company credit ratings.