Derivative Financial Instruments Derivative Financial Instruments - Narrative (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Aggregate fair value of derivative instruments with credit-risk-related contingent features, in case of net liability position	$ 81,632,000	$ 38,324,000
Collateral related to the derivatives	16,171,000	0
Fair value of assets required to settle the derivatives in, net liability position	65,461,000
Additional collateral to satisfy collateral netting arrangements	25,353,000	103,214,000
Net derivative gains estimated to be reclassified from accumulated other comprehensive income (loss) into net income within the next twelve months	10,544,000
Derivative financial instruments
Notional amount	5,516,020,000	2,396,297,000
Net derivatives, Fair value	15,837,000	108,104,000
Asset derivatives, Fair value	98,777,000	146,385,000
Liability derivatives, Fair value	82,940,000	38,281,000
Interest rate swaps
Derivative financial instruments
Average notional amount of the derivative instruments	908,500,000	784,900,000
Cross-currency swaps
Derivative financial instruments
Average notional amount of the derivative instruments	1,302,507,000	1,141,967,000
Future
Derivative financial instruments
Average notional amount of the derivative instruments	113,004,000	145,658,000
Swaption
Derivative financial instruments
Average notional amount of the derivative instruments	162,408,000	156,632,000
Other forward contracts
Derivative financial instruments
Average notional amount of the derivative instruments	2,409,855,000	2,230,167,000
Derivatives designated as hedges:
Derivative financial instruments
Notional amount	1,175,690,000	1,034,008,000
Net derivatives, Fair value	25,172,000	78,737,000
Asset derivatives, Fair value	60,356,000	87,031,000
Liability derivatives, Fair value	35,184,000	8,294,000
Derivatives designated as hedges: | Cash flow hedges:
Derivative financial instruments
Notional amount	1,175,690,000	1,034,008,000
Net derivatives, Fair value	25,172,000	78,737,000
Asset derivatives, Fair value	60,356,000	87,031,000
Liability derivatives, Fair value	35,184,000	8,294,000
Derivatives designated as hedges: | Cash flow hedges: | Interest rate swaps
Derivative financial instruments
Notional amount	419,800,000	419,800,000
Net derivatives, Fair value	29,482,000	33,390,000
Asset derivatives, Fair value	29,482,000	33,390,000
Liability derivatives, Fair value	0	0
Derivatives designated as hedges: | Cash flow hedges: | Cross-currency swaps
Derivative financial instruments
Notional amount	755,890,000	614,208,000
Net derivatives, Fair value	(4,310,000)	45,347,000
Asset derivatives, Fair value	30,874,000	53,641,000
Liability derivatives, Fair value	35,184,000	8,294,000
Derivatives not designated as hedges:
Derivative financial instruments
Notional amount	4,340,330,000	1,362,289,000
Net derivatives, Fair value	(9,335,000)	29,367,000
Asset derivatives, Fair value	38,421,000	59,354,000
Liability derivatives, Fair value	47,756,000	29,987,000
Derivatives not designated as hedges: | Interest rate swaps
Derivative financial instruments
Notional amount	507,100,000	468,100,000
Net derivatives, Fair value	(2,477,000)	(4,358,000)
Asset derivatives, Fair value	10,147,000	8,982,000
Liability derivatives, Fair value	12,624,000	13,340,000
Derivatives not designated as hedges: | Cross-currency swaps
Derivative financial instruments
Notional amount	612,733,000	612,733,000
Net derivatives, Fair value	(3,863,000)	33,371,000
Asset derivatives, Fair value	27,193,000	50,018,000
Liability derivatives, Fair value	31,056,000	16,647,000
Derivatives not designated as hedges: | Futures on equity indices
Derivative financial instruments
Notional amount	29,674,000	34,422,000
Net derivatives, Fair value	0	0
Asset derivatives, Fair value	0	0
Liability derivatives, Fair value	0	0
Derivatives not designated as hedges: | Interest rate futures
Derivative financial instruments
Notional amount	74,200,000	81,500,000
Net derivatives, Fair value	0	0
Asset derivatives, Fair value	0	0
Liability derivatives, Fair value	0	0
Derivatives not designated as hedges: | Interest rate swaptions
Derivative financial instruments
Notional amount	162,423,000	165,534,000
Net derivatives, Fair value	156,000	354,000
Asset derivatives, Fair value	156,000	354,000
Liability derivatives, Fair value	0	$ 0
Derivatives not designated as hedges: | Other forward contracts
Derivative financial instruments
Notional amount	2,954,200,000
Net derivatives, Fair value	(3,151,000)
Asset derivatives, Fair value	925,000
Liability derivatives, Fair value	$ 4,076,000