Debt Debt Line of Credit and Commercial Paper (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.00%
Unamortized Discount	$ 46,260
A 175 billion LOC [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility	1,750,000
A 250 billion LOC [Member] [Domain]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility	$ 2,500,000
Debt Instrument, Maturity Date	Apr. 01, 2018
500.0 million ability to increase	$ 500,000
Line Of Credit Facility Commitment Fee	15 Basis Points
Remaining borrowing capacity	$ 2,070,000	$ 2,120,000
Amount restricted/dedicated to support letters of credit	$ 45,100	$ 43,800
Weighted average interest rate on revolving letter of credit	1.07%	0.95%
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Commercial paper, maximum borrowing capacity	$ 500,000
Commercial Paper, net	387,300
commercial paper principal outstanding	387,500
Unamortized Discount	$ 200
Weighted Average Interest Rate Commercial Paper	0.56%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Long-term Line of Credit		$ 333,000
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member] | A 250 billion LOC [Member] [Domain]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR plus 0.95%