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Fair Value Measurements - Summary of Changes in Level 3 Fair Value Category (Detail) - Successor [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Investment by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 808,557		$ 762,703
Included in earnings	35,184		9,819
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(52,508)		6,285
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases			9,851
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales					$ (79,319)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(713,603)		(9,793)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three	0		22,078
Balance, end of period	77,630	$ 808,557	721,624	$ 762,703
Derivatives by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	60,808		(100,282)
Included in earnings	(9,375)		(2,817)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues		0			0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(1,525)		511
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three	0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Out Of Level Three	0		0
Balance, end of period	49,908	60,808	(102,588)	(100,282)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	9,481		3,017
Non-Agency RMBS [Member] | Investment by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	736,017		696,713
Included in earnings	34,727		9,466
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(52,138)		6,564
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases			9,851
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(713,297)		(9,536)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three			22,078
Balance, end of period	5,309	736,017	655,817	696,713
Residential Mortgage-Backed Securities [Member] | Investment by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales					(79,319)
Asset-backed Securities [Member] | Investment by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	72,540		65,990
Included in earnings	457		353
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(370)		(279)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(306)		(257)
Balance, end of period	72,321	72,540	65,807	65,990
Interest Rate Swap [Member] | Interest Rate Swap [Member] | Derivatives by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	61,374		(84,933)
Included in earnings	(9,029)		(3,869)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues		0			0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(1,419)		710
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three	0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Out Of Level Three	0		0
Balance, end of period	50,926	61,374	(88,092)	(84,933)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	9,029		3,869
Credit Derivative [Member] | Credit Derivative [Member] | Derivatives by Class [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	(566)		(15,349)
Included in earnings	(346)		1,052
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues		0			$ 0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales		0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(106)		(199)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three	0		0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Out Of Level Three	0		0
Balance, end of period	(1,018)	(566)	(14,496)	(15,349)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	452		(852)
Level 3 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	12,561,185		11,369,112
Included in earnings	(86,178)		253,249
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	378,151		189,925
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases			9,851
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales			(79,319)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(902,421)		(176,439)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three	0		22,078
Balance, end of period	11,950,737	12,561,185	11,588,457	11,369,112
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	121,468		(243,230)
Level 3 [Member] | Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	808,557		762,703
Included in earnings	35,184		9,819
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(52,508)		6,285
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases			9,851
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales			(79,319)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(713,603)		(9,793)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Transfers Into Level Three	0		22,078
Balance, end of period	77,630	808,557	721,624	762,703
Level 3 [Member] | Other Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	5,979		7,382
Included in earnings	(358)		(343)
Balance, end of period	5,621	5,979	7,039	7,382
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	358		343
Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	60,808		(100,282)
Included in earnings	(9,375)		(2,817)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(1,525)		511
Balance, end of period	49,908	60,808	(102,588)	(100,282)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	9,481		3,017
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Long-term Debt [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	(2,757,688)		(2,582,220)
Included in earnings	159,985		(94,407)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(103,877)		(46,257)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	5,064		6,242
Balance, end of period	(2,696,516)	(2,757,688)	(2,716,642)	(2,582,220)
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	(159,985)		94,407
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Loans Receivable [Member] | Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	11,529,384		10,658,963
Included in earnings	(202,588)		346,192
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	423,892		183,549
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(192,357)		(173,399)
Balance, end of period	11,558,331	11,529,384	11,015,305	10,658,963
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	202,588		(346,192)
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Corporate Debt Securities [Member] | Investments Contract [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	2,914,145		2,622,566
Included in earnings	(69,026)		(5,195)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	110,644		46,348
Balance, end of period	2,955,763	$ 2,914,145	2,663,719	$ 2,622,566
The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date	$ 69,026		$ 5,195

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