Derivative Financial Instruments - Notional Amount and FV of Derivative Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Derivative financial instruments
Notional amount		$ 2,567,989	$ 2,396,297
Net derivatives, Fair value		(43,184)	108,104
Asset derivatives, Fair value	[1]	51,359	146,385
Liability derivatives, Fair value	[1]	94,543	38,281
Derivatives designated as hedges
Derivative financial instruments
Notional amount		1,188,860	1,034,008
Net derivatives, Fair value		(23,882)	78,737
Asset derivatives, Fair value	[1]	27,434	87,031
Liability derivatives, Fair value	[1]	51,316	8,294
Derivatives designated as hedges | Cash flow hedges
Derivative financial instruments
Notional amount		1,188,860	1,034,008
Net derivatives, Fair value		(23,882)	78,737
Asset derivatives, Fair value	[1]	27,434	87,031
Liability derivatives, Fair value	[1]	51,316	8,294
Derivatives designated as hedges | Cash flow hedges | Interest rate swaps
Derivative financial instruments
Notional amount		388,800	419,800
Net derivatives, Fair value		7,476	33,390
Asset derivatives, Fair value	[1]	7,476	33,390
Derivatives designated as hedges | Cash flow hedges | Cross-currency swaps
Derivative financial instruments
Notional amount		800,060	614,208
Net derivatives, Fair value		(31,358)	45,347
Asset derivatives, Fair value	[1]	19,958	53,641
Liability derivatives, Fair value	[1]	51,316	8,294
Derivatives not designated as hedges
Derivative financial instruments
Notional amount		1,379,129	1,362,289
Net derivatives, Fair value		(19,302)	29,367
Asset derivatives, Fair value	[1]	23,925	59,354
Liability derivatives, Fair value	[1]	43,227	29,987
Derivatives not designated as hedges | Interest rate swaps
Derivative financial instruments
Notional amount		519,100	468,100
Net derivatives, Fair value		1,902	(4,358)
Asset derivatives, Fair value	[1]	3,530	8,982
Liability derivatives, Fair value	[1]	1,628	13,340
Derivatives not designated as hedges | Cross-currency swaps
Derivative financial instruments
Notional amount		612,733	612,733
Net derivatives, Fair value		(21,279)	33,371
Asset derivatives, Fair value	[1]	20,320	50,018
Liability derivatives, Fair value	[1]	41,599	16,647
Derivatives not designated as hedges | Futures on equity indices
Derivative financial instruments
Notional amount		22,074	34,422
Derivatives not designated as hedges | Interest rate futures
Derivative financial instruments
Notional amount		60,700	81,500
Derivatives not designated as hedges | Interest rate swaptions
Derivative financial instruments
Notional amount		164,522	165,534
Net derivatives, Fair value		75	354
Asset derivatives, Fair value	[1]	$ 75	$ 354

[1] The estimated fair value excludes accrued income and expense. The estimated fair value of all derivatives in an asset position is reported within other assets and the estimated fair value of all derivatives in a liability position is reported within other liabilities in the consolidated balance sheets.