Derivative Financial Instruments and Hedging Activities - Fixed-to-Floating Interest Rate Swaps (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Notes due 2017 [Member]
Derivative [Line Items]
Notional amount	$ 150
Fixed interest rate received (percentage)	5.30%
Floating interest rate paid (percentage)	4.75%
Senior notes due 2017 [Member]
Derivative [Line Items]
Notional amount	$ 750
Fixed interest rate received (percentage)	1.50%
Floating interest rate paid (percentage)	0.95%
Notes due 2018 [Member]
Derivative [Line Items]
Notional amount	$ 415
Fixed interest rate received (percentage)	5.60%
Floating interest rate paid (percentage)	4.26%
Notes due 2019 [Member]
Derivative [Line Items]
Notional amount	$ 300
Fixed interest rate received (percentage)	6.95%
Floating interest rate paid (percentage)	5.85%
Debentures due 2019 [Member]
Derivative [Line Items]
Notional amount	$ 25
Fixed interest rate received (percentage)	8.88%
Floating interest rate paid (percentage)	4.96%
Debentures due 2020 [Member]
Derivative [Line Items]
Notional amount	$ 150
Fixed interest rate received (percentage)	3.875%
Floating interest rate paid (percentage)	2.61%
Notes due 2021 [Member]
Derivative [Line Items]
Notional amount	$ 275
Fixed interest rate received (percentage)	3.47%
Floating interest rate paid (percentage)	2.23%
Senior notes due 2022 [Member]
Derivative [Line Items]
Notional amount	$ 1,400
Fixed interest rate received (percentage)	2.75%
Floating interest rate paid (percentage)	1.07%
Notes due 2023 [Member]
Derivative [Line Items]
Notional amount	$ 200
Fixed interest rate received (percentage)	3.68%
Floating interest rate paid (percentage)	1.56%
Debentures due 2024 [Member]
Derivative [Line Items]
Notional amount	$ 25
Fixed interest rate received (percentage)	7.625%
Floating interest rate paid (percentage)	3.55%
Debentures due 2029 [Member]
Derivative [Line Items]
Notional amount	$ 50
Fixed interest rate received (percentage)	7.65%
Floating interest rate paid (percentage)	3.65%
Debentures due 2034 [Member]
Derivative [Line Items]
Notional amount	$ 25
Fixed interest rate received (percentage)	5.45%
Floating interest rate paid (percentage)	1.36%
London Interbank Offered Rate (LIBOR) [Member]
Derivative [Line Items]
Debt instrument, term (in years)	3 months
Basis spread on variable rate (percentage)	0.00%
London Interbank Offered Rate (LIBOR) [Member] | Notes due 2017 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	1 month
Basis spread on variable rate (percentage)	4.26%
London Interbank Offered Rate (LIBOR) [Member] | Senior notes due 2017 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	1 month
Basis spread on variable rate (percentage)	0.46%
London Interbank Offered Rate (LIBOR) [Member] | Notes due 2018 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	6 months
Basis spread on variable rate (percentage)	3.18%
London Interbank Offered Rate (LIBOR) [Member] | Notes due 2019 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	3 months
Basis spread on variable rate (percentage)	5.07%
London Interbank Offered Rate (LIBOR) [Member] | Debentures due 2019 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	6 months
Basis spread on variable rate (percentage)	3.84%
London Interbank Offered Rate (LIBOR) [Member] | Debentures due 2020 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	1 month
Basis spread on variable rate (percentage)	2.12%
London Interbank Offered Rate (LIBOR) [Member] | Notes due 2021 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	1 month
Basis spread on variable rate (percentage)	1.74%
London Interbank Offered Rate (LIBOR) [Member] | Senior notes due 2022 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	1 month
Basis spread on variable rate (percentage)	0.58%
London Interbank Offered Rate (LIBOR) [Member] | Notes due 2023 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	1 month
Basis spread on variable rate (percentage)	1.07%
London Interbank Offered Rate (LIBOR) [Member] | Debentures due 2024 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	6 months
Basis spread on variable rate (percentage)	2.48%
London Interbank Offered Rate (LIBOR) [Member] | Debentures due 2029 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	6 months
Basis spread on variable rate (percentage)	2.57%
London Interbank Offered Rate (LIBOR) [Member] | Debentures due 2034 [Member]
Derivative [Line Items]
Debt instrument, term (in years)	6 months
Basis spread on variable rate (percentage)	0.28%