Derivatives Derivatives, Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative, Notional Amount	$ 8,430,000	$ 9,680,000
Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	1,705,000	2,955,000
Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	425,000	$ 425,000
Interest Rate Swap Agreements, Maturities in Next 13-24 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 185,000
Derivative, Average Fixed Interest Rate	0.92%
Interest Rate Swap Agreements, Maturities in Next 13-24 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 25-36 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 125,000
Derivative, Average Fixed Interest Rate	1.40%
Interest Rate Swap Agreements, Maturities in Next 25-36 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 470,000
Derivative, Average Fixed Interest Rate	1.74%
Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250,000
Derivative, Average Fixed Interest Rate	1.91%
Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 250,000
Derivative, Average Fixed Interest Rate	1.58%
Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 150,000
Derivative, Average Fixed Interest Rate	1.71%
Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 75,000
Derivative, Average Fixed Interest Rate	1.77%
Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 125,000
Derivative, Average Fixed Interest Rate	1.98%
Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 85-96 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 85-96 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%
Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 25,000
Derivative, Average Fixed Interest Rate	2.71%
Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member] | Interest Rate Swap [Member] | Pay-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 475,000
Derivative, Average Fixed Interest Rate	2.86%
Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member] | Interest Rate Swap [Member] | Receive-fixed interest rate swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 0
Derivative, Average Fixed Interest Rate	0.00%