Derivative Instruments - Summary of Gross Fair Values of Individual Derivative Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 59,768	$ 73,826
Derivative Asset, Fair Value, Amount Offset Against Collateral	300	627
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	59,468	73,199
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	76,699	82,782
Fair value of derivative liabilities	70,505
Net Amount, Derivative Assets	59,468	73,199
Net Amount, Derivative Liabilities	6,194	1,522
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Derivative Liability, Fair Value, Gross Liability	76,999	83,409
Derivative Liability, Fair Value, Amount Offset Against Collateral	300	627
Variable Interest Entity, Primary Beneficiary [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	66,302	54,877
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	66,302	54,877
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,712,062	2,205,264
Fair value of derivative liabilities	0
Net Amount, Derivative Assets	66,302	54,877
Net Amount, Derivative Liabilities	1,712,062	2,205,264
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Derivative Liability, Fair Value, Gross Liability	1,712,062	2,205,264
Derivative Liability, Fair Value, Amount Offset Against Collateral	0
Other Credit Derivatives [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,459
Net Amount, Derivative Liabilities	1,459	566
Derivative Liability, Fair Value, Gross Liability	1,459	566
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	59,768	73,826
Derivative Asset, Fair Value, Amount Offset Against Collateral	300	627
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	59,468	73,199
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	71,861
Fair value of derivative liabilities	67,126	79,912
Net Amount, Derivative Assets	59,468	73,199
Net Amount, Derivative Liabilities	4,735	956
Derivative Liability, Fair Value, Gross Liability	72,161	81,495
Derivative Liability, Fair Value, Amount Offset Against Collateral	300
Interest Rate Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,712,062
Net Amount, Derivative Liabilities	1,712,062	2,205,264
Derivative Liability, Fair Value, Gross Liability	1,712,062	2,205,264
Future [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	3,379
Fair value of derivative liabilities	3,379	1,348
Net Amount, Derivative Liabilities	0	0
Derivative Liability, Fair Value, Gross Liability	3,379	1,348
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	66,302	54,877
Derivative Asset, Fair Value, Amount Offset Against Collateral	0
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	66,302	54,877
Net Amount, Derivative Assets	$ 66,302	$ 54,877