Derivative Financial Instruments - Impact of Derivatives on Consolidated Statements of Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Interest rate swaps:
Total	$ 825	$ 5,401	$ (9,458)	$ (764)	$ (215)	$ 297	$ (52)	$ 610	$ 7,892		$ 29,308		$ 30,527
Interest Rate Swaps | Fair Value
Interest rate swaps:
Hedge ineffectiveness gains (losses) recorded in earnings									1,718		(558)		(6,061)
Realized gains recorded in interest expense									20,958		28,668		35,988
Total									22,676		28,110		29,927
Interest Rate Swaps | Cash Flow
Interest rate swaps:
Total									(9,590)		0		0
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net									(520)		0		0
Gain (Loss) from Components Excluded from Assessment of Cash Flow Hedge Effectiveness, Net									(9,070)		0		0
Interest Rate Swaps | Trading
Interest rate swaps:
Interest reclassification									(2,250)		1,285		87
Change in fair value of future interest payments recorded in earnings									(2,944)		(87)		513
Total									$ (5,194)	[1]	$ 1,198	[1]	$ 600	[1]

[1] Amounts included in "gains (losses) on derivatives and hedging activities, net."