Derivative Instruments and Hedging Activities Schedule of Constant Maturity Swaps (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative, Notional Amount	$ 57,015,981	$ 43,379,217	$ 37,366,610	$ 36,472,401	$ 44,262,605	$ 22,377,656
Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	18,000,000	6,000,000	10,000,000	8,000,000	19,000,000	0
Derivative, Fair Value, Net	190		(3,773)
Derivative, Cost	0		0
Derivative, Unrealized Gains (Losses)	190		(3,773)
Derivative, Average Strike Swap Rate	0.552%		0.821%
Derivative Financial Instruments, Assets [Member]
Derivative [Line Items]
Derivative, Notional Amount	60,460,981		30,383,908
Derivative, Fair Value, Net	353,893		549,859
Derivative Financial Instruments, Assets [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	18,000,000		0
Derivative, Fair Value, Net	190		0
Determination Date, February 2014 [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount			3,000,000
Derivative, Fair Value, Net			625
Derivative, Cost			0
Derivative, Unrealized Gains (Losses)			625
Derivative, Average Strike Swap Rate			0.768%
Determination Date, March 2014 [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount			5,000,000
Derivative, Fair Value, Net			(3,171)
Derivative, Cost			0
Derivative, Unrealized Gains (Losses)			(3,171)
Derivative, Average Strike Swap Rate			0.85%
Determination Date, June 2014 [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount			2,000,000
Derivative, Fair Value, Net			(1,227)
Derivative, Cost			0
Derivative, Unrealized Gains (Losses)			(1,227)
Derivative, Average Strike Swap Rate			0.828%
Determination Date, October 2014 [Member]
Derivative [Line Items]
Derivative, Notional Amount	5,000,000
Derivative, Fair Value, Net	(527)
Derivative, Average Strike Swap Rate	0.563%
Determination Date, October 2014 [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Cost	0
Derivative, Unrealized Gains (Losses)	(527)
Determination Date, November 2014 [Member]
Derivative [Line Items]
Derivative, Notional Amount	2,000,000
Derivative, Fair Value, Net	(677)
Derivative, Cost	0
Derivative, Unrealized Gains (Losses)	(677)
Derivative, Average Strike Swap Rate	0.591%
Determination Date, December 2014 [Member]
Derivative [Line Items]
Derivative, Notional Amount	7,000,000
Derivative, Fair Value, Net	323
Derivative, Average Strike Swap Rate	0.552%
Determination Date, December 2014 [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Cost	0
Derivative, Unrealized Gains (Losses)	323
Determination Date, January 2015 [Member]
Derivative [Line Items]
Derivative, Notional Amount	4,000,000
Derivative, Fair Value, Net	1,071
Derivative, Average Strike Swap Rate	0.518%
Determination Date, January 2015 [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Cost	0
Derivative, Unrealized Gains (Losses)	$ 1,071