Derivative Financial Instruments (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£) item	Dec. 31, 2015 USD ($) item	Sep. 29, 2015 GBP (£)	Jan. 12, 2015 GBP (£)	Jan. 12, 2015 USD ($)
Derivative
Semi-annual buy (sell) amount					£ 1,500	$ 1,000,000
Exchange rate GBP/USD					1.5149	1.5149
Earned additional interest income resulting from cash flow hedges	$ 100,000
Additional interest expense resulting from cash flow hedges	4,000,000
Reclassification out of Accumulated Other Comprehensive Income
Effects of Change in Interest Rates
Reclassification of unrealized gains into other income (expense), discontinuation	500,000
Net Investment Hedging
Derivative
Notional amount | £				£ 268,000
Interest rate swap, entered in July 2005, maturity in July 2020
Effects of Change in Interest Rates
+50 Basis Points	1,034,000
-50 Basis Points	(933,000)
+100 Basis Points	2,017,000
-100 Basis Points	(1,916,000)
Interest rate swap, entered in July 2005, maturity in July 2020 | BMA Swap Index
Derivative
Notional amount			$ 45,600,000
Fair value of hedge, liabilities			$ (5,430,000)
Interest rate swap, entered in July 2005, maturity in July 2020 | Cash flow hedge
Derivative
Number of interest-rate swap contracts | item		3	3
Interest rate swap, entered in July 2005, maturity in July 2020 | Cash flow hedge | BMA Swap Index
Derivative
Fixed Rate/Buy Amount (as a percent)		3.82%	3.82%
Interest rate swap, entered in November 2008, maturity in October 2016
Effects of Change in Interest Rates
+50 Basis Points	102,000
-50 Basis Points	(96,000)
+100 Basis Points	201,000
-100 Basis Points	(195,000)
Interest rate swap, entered in November 2008, maturity in October 2016 | LIBOR
Derivative
Notional amount			$ 25,100,000
Fair value of hedge, liabilities			$ (761,000)
Interest rate swap, entered in November 2008, maturity in October 2016 | Cash flow hedge | LIBOR
Derivative
Fixed Rate/Buy Amount (as a percent)		5.95%	5.95%
Floating/Exchange Rate Index, percentage		1.50%	1.50%
Interest rate swap, entered in July 2012, maturity in June 2016
Effects of Change in Interest Rates
+50 Basis Points	469,000
-50 Basis Points	(452,000)
+100 Basis Points	937,000
-100 Basis Points	(906,000)
Interest rate swap, entered in July 2012, maturity in June 2016 | GBP LIBOR
Derivative
Notional amount | £		£ 137,000
Fair value of hedge, liabilities			$ (60,000)
Interest rate swap, entered in July 2012, maturity in June 2016 | Cash flow hedge
Derivative
Buy (sell) amount | £		£ 7,200
Interest rate swap, entered in July 2012, maturity in June 2016 | Cash flow hedge | GBP LIBOR
Derivative
Fixed Rate/Buy Amount (as a percent)		1.81%	1.81%
Floating/Exchange Rate Index, percentage		1.20%	1.20%
Interest rate swap, entered in January 2015, maturity in October 2017
Derivative
Notional amount | £					£ 220,000
Effects of Change in Interest Rates
+50 Basis Points	2,982,000
-50 Basis Points	(2,916,000)
+100 Basis Points	5,931,000
-100 Basis Points	(5,866,000)
Interest rate swap, entered in January 2015, maturity in October 2017 | GBP LIBOR
Derivative
Notional amount | £		£ 220,000
Fair value of interest rate hedge, assets			$ 196,000
Interest rate swap, entered in January 2015, maturity in October 2017 | Cash flow hedge
Derivative
Buy (sell) amount | £		£ 1,000
Semi-annual buy (sell) amount			$ 1.5149
Interest rate swap, entered in January 2015, maturity in October 2017 | Cash flow hedge | GBP LIBOR
Derivative
Fixed Rate/Buy Amount (as a percent)		1.79%	1.79%
Floating/Exchange Rate Index, percentage		0.975%	0.975%
Currency swap, entered in July 2012, maturity in June 2016
Derivative
Notional amount | £		£ 7,200
Fair value of foreign currency derivative instruments not designated as hedging instruments, assets			$ 685,000
Effects of Change in Interest Rates
+50 Basis Points	(44,000)
-50 Basis Points	63,000
+100 Basis Points	(97,000)
-100 Basis Points	116,000
Currency swap, entered in July 2012, maturity in June 2016 | Cash flow hedge
Derivative
Notional amount | £		137,000
Semi-annual buy (sell) amount		£ 7,000	$ 11,400,000
Exchange rate GBP/USD		1.5695	1.5695
Currency swap, entered in July 2012, maturity in June 2016 | Buy | Cash flow hedge
Derivative
Semi-annual buy (sell) amount			$ 11,000,000
Currency swap, entered in January 2015, maturity in October 2017
Derivative
Notional amount | £		£ 23,200
Fair value of hedge, liabilities			866,000
Effects of Change in Interest Rates
+50 Basis Points	(95,000)
-50 Basis Points	246,000
+100 Basis Points	(265,000)
-100 Basis Points	$ 417,000
Currency swap, entered in January 2015, maturity in October 2017 | Cash flow hedge
Derivative
Semi-annual buy (sell) amount			$ 35,100,000