Business Combinations and Assumption of Insured Portfolio - Assets and Liabilities Assumed (Details) - MBIA UK [Member] $ in Millions	Jan. 10, 2017 USD ($)
Business Acquisition [Line Items]
Cash purchase price	$ 334
Identifiable assets acquired:
Investments	459
Cash	72
Premiums receivable, net of commissions payable, fair value of net assets acquired, before settlement of pre-existing relationships	274
Premiums receivable, net of commissions payable, net of settlement of pre-existing relationships	(4)
Premiums receivable, net of commissions payable, net effect of MBIA UK Acquisition	270
Other assets	16
Other assets, net of settlement of pre-existing relationships	(6)
Other assets, Net effect of MBIA UK Acquisition	10
Total assets	821
Total assets, net of settlement of pre-existing relationships	(10)
Total assets, Net effect of MBIA UK Acquisition	811
Liabilities assumed:
Unearned premium reserves, fair value of net assets acquired, before settlement of pre-existing relationships	389
Unearned premium reserves, net of settlement of pre-existing relationships	(6)
Unearned premium reserves, Net effect of MBIA UK Acquisition	383
Current tax payable, fair value of net assets acquired, before settlement of pre-existing relationships	25
Current tax payable, net of settlement of pre-existing relationships	0
Current tax payable, Net effect of MBIA UK Acquisition	25
Other liabilities, fair value of net assets acquired, before settlement of pre-existing relationships	4
Other liabilities, net of settlement of pre-existing relationships	(5)
Other liabilities, Net effect of MBIA UK Acquisition	(1)
Total liabilities	418
Total liabilities, net of settlement of pre-existing relationships	(11)
Total liabilities, Net effect of MBIA UK Acquisition	407
Net assets of MBIA UK, fair value of net assets acquired, before settlement of pre-existing relationships	403
Net assets of MBIA UK, net of settlement of pre-existing relationships	1
Net assets of MBIA UK, Net effect of MBIA UK Acquisition	404
Cash acquired from MBIA Holdings, fair value of net assets acquired, before settlement of pre-existing relationships	23
Cash acquired from MBIA Holdings, net of settlement of pre-existing relationships	0
Cash acquired from MBIA Holdings, Net effect of MBIA UK Acquisition	23
Deferred tax liability, fair value of net assets acquired, before settlement of pre-existing relationships	(36)
Deferred tax liability, net of settlement of pre-existing relationships	0
Deferred tax liability, Net effect of MBIA UK Acquisition	(36)
Net assets of acquisition	390
Net assets effect of MBIA UK Acquisition, net of settlement of pre-existing relationships	1
Net asset effect of MBIA UK Acquisition, Net effect of MBIA UK Acquisition	391
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after tax, fair value of net assets acquired, before settlement of pre-existing relationships	56
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, net of settlement of pre-existing relationships	1
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, Net effect of MBIA UK Acquisition	57
Deferred tax, net of settlement of pre-existing relationships	1
Deferred tax, Net effect of MBIA UK Acquisition	1
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisitions, pre-tax, fair value of net assets acquired, before settlement of pre-existing relationships	56
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, net of settlement of pre-existing relationships	2
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, Net effect of MBIA UK Acquisition	58
Assumed assets including pre-existing relationship	385
Settlement of pre-existing relationship	5
Cash purchase price	$ 329