Derivative Financial Instruments (Details)	3 Months Ended								12 Months Ended							12 Months Ended		12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 Cash flow hedge USD ($)	Dec. 31, 2014 Interest rate swap, entered in July 2005, maturity in July 2020 USD ($)	Dec. 31, 2014 Interest rate swap, entered in July 2005, maturity in July 2020 Cash flow hedge item	Dec. 31, 2014 Interest rate swap, entered in July 2005, maturity in July 2020 Cash flow hedge BMA Swap Index USD ($)	Dec. 31, 2014 Interest rate swap, entered in November 2008, maturity in October 2016 USD ($)	Dec. 31, 2014 Interest rate swap, entered in November 2008, maturity in October 2016 Cash flow hedge LIBOR USD ($)	Dec. 31, 2014 Interest rate swap, entered in July 2012, maturity in June 2016 USD ($)	Dec. 31, 2014 Interest rate swap, entered in July 2012, maturity in June 2016 Cash flow hedge GBP (£)	Dec. 31, 2014 Interest rate swap, entered in July 2012, maturity in June 2016 Cash flow hedge GBP LIBOR USD ($)	Dec. 31, 2014 Currency swap, entered in July 2012, maturity in June 2016 USD ($)	Dec. 31, 2014 Currency swap, entered in July 2012, maturity in June 2016 Cash flow hedge USD ($)	Dec. 31, 2014 Currency swap, entered in July 2012, maturity in June 2016 Cash flow hedge GBP (£)	Dec. 31, 2014 Currency swap, entered July 2014, maturity in December 2015 USD ($)	Dec. 31, 2014 Currency swap, entered July 2014, maturity in December 2015 Cash flow hedge USD ($)	Dec. 31, 2014 Currency swap, entered July 2014, maturity in December 2015 Cash flow hedge GBP (£)
Derivative
Buy (sell) amount																			£ 7,200,000							£ 400,000
Notional amount																							137,000,000
Semi-annual buy (sell) amount																						11,000,000	7,000,000	600,000
Fair value of hedge, liabilities															(5,939,000)		(1,724,000)
Fair value of interest rate hedge, assets																				178,000		200,000
Fair value of foreign currency hedge, assets																						276,000			653,000
Number of interest-rate swap contracts														3
Exchange rate USD/GBP																						1.5695			1.7060	1.7060
Gain (loss) on cash flow hedge, ineffective portion																						0		0
Earned additional interest income resulting from cash flow hedges									1,000,000
Additional interest expense resulting from cash flow hedges									4,000,000
Gains or losses recorded to accumulated other comprehensive loss reclassified to earnings	199,630,000	251,059,000	222,279,000	263,623,000	297,639,000	236,858,000	216,725,000	233,784,000	936,591,000	985,006,000	846,842,000
Effects of Change in Interest Rates
+50 Basis Points													1,170,000			235,000		1,533,000			(189,000)			(38,000)
-50 Basis Points													(1,194,000)			(220,000)		(1,560,000)			149,000			30,000
+100 Basis Points													2,351,000			462,000		3,079,000			(358,000)			(72,000)
-100 Basis Points													(2,375,000)			(447,000)		(3,106,000)			319,000			64,000
Reclassification of unrealized gains into other income (expense)												$ 2,200,000