Derivative Assets And Liabilities Outstanding Interest Rate Derivatives (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
July 2016 [Member]
Notional Amount	[1]	$ 200	$ 200
Type	[1],[2]	Forward-starting to pay a fixed rate of 3.80% and receive a floating rate
July 2017 [Member]
Notional Amount	[3]	$ 300	300
Type	[2],[3]	Forward-starting to pay a fixed rate of 3.84% and receive a floating rate
July 2018 [Member]
Notional Amount	[3]	$ 200	200
Type	[2],[3]	Forward-starting to pay a fixed rate of 4.00% and receive a floating rate
July 2019 [Member]
Notional Amount	[3]	$ 200	200
Type	[2],[3]	Forward-starting to pay a fixed rate of 3.25% and receive a floating rate
December 2018 [Member]
Notional Amount		$ 1,200	1,200
Type	[2]	Pay a floating rate based on a 3-month LIBOR and receive a fixed rate of 1.53%
March 2019 [Member]
Notional Amount		$ 300	$ 300
Type	[2]	Pay a floating rate based on a 3-month LIBOR and receive a fixed rate of 1.42%

[1] Represents the effective date. These forward-starting swaps have terms of 10 and 30 years with a mandatory termination date the same as the effective date.

[2] Floating rates are based on 3-month LIBOR.

[3] Represents the effective date. These forward-starting swaps have terms of 30 years with a mandatory termination date the same as the effective date.