Acquisitions - Assets and Liabilities Assumed (Details) - USD ($) $ in Millions	Jan. 10, 2017	Jul. 01, 2016	Apr. 01, 2015
Radian [Member]
Business Acquisition [Line Items]
Cash purchase price			$ 804
Identifiable assets acquired:
Investments			1,473
Cash			4
Ceded unearned premium reserve, fair value of net assets acquired, before settlement of pre-existing relationships			(3)
Ceded unearned premium reserve, net effect of settlement of pre-existing relationships			(65)
Ceded unearned premium reserve, net effect of Radian asset acquisition			(68)
Credit derivative assets			30
Deferred tax assets, net, fair value of net assets acquired, before settlement of pre-existing relationships			263
Deferred tax asset, net effect of settlement of pre-exiting relationships			(56)
Deferred tax asset, net effect of CIFG acquisition			207
Financial guaranty variable interest entities' assets			122
Other assets, fair value of net assets acquired, before settlement of pre-existing relationships			86
Other assets, net of settlement of pre-existing relationships			(67)
Other assets, Net effect of MBIA UK Acquisition			19
Total assets, fair value of net assets acquired, before settlement of pre-existing relationships			1,975
Total assets, net of settlement of pre-existing relationships			(188)
Total assets, Net effect of MBIA UK Acquisition			1,787
Liabilities assumed:
Unearned premium reserves, fair value of net assets acquired, before settlement of pre-existing relationships			697
Unearned premium reserves, net of settlement of pre-existing relationships			(216)
Unearned premium reserves, Net effect of MBIA UK Acquisition			481
Credit derivative liabilities, fair value of net assets acquired before settlement of pre-existing relationships			271
Credit derivative liabilities, net effect of settlement of pre-existing relationships			(26)
Credit derivative liabilities, net effect of Radian asset acquisition			245
Financial guaranty variable interest entities' liabilities			118
Other liabilities, fair value of net assets acquired, before settlement of pre-existing relationships			30
Other liabilities, net of settlement of pre-existing relationships			(49)
Other liabilities, Net effect of MBIA UK Acquisition			(19)
Total liabilities, fair value of net assets acquired, before settlement of pre-existing relationships			1,116
Total liabilities, net of settlement of pre-existing relationships			(291)
Total liabilities, Net effect of MBIA UK Acquisition			825
Net assets effect of MBIA UK Acquisition, fair value of net assets acquired, before settlement of pre-existing relationships			859
Net assets effect of MBIA UK Acquisition, net of settlement of pre-existing relationships			103
Net asset effect of MBIA UK Acquisition, Net effect of MBIA UK Acquisition			962
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			55
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, net of settlement of pre-existing relationships			103
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, Net effect of MBIA UK Acquisition			158
Deferred tax, net of settlement of pre-existing relationships			56
Deferred tax, Net effect of MBIA UK Acquisition			56
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			55
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, net of settlement of pre-existing relationships			159
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, Net effect of MBIA UK Acquisition			214
Assumed assets including pre-existing relationship			1,042
Settlement of pre-existing relationship			(183)
Consideration transferred			$ 987
CIFG Holding Inc. [Member]
Business Acquisition [Line Items]
Cash purchase price		$ 443
Identifiable assets acquired:
Investments		770
Cash		8
Premiums receivable, net of commissions payable, fair value of net assets acquired, before settlement of pre-existing relationships		18
Ceded unearned premium reserve, fair value of net assets acquired, before settlement of pre-existing relationships		173
Ceded unearned premium reserve, net effect of settlement of pre-existing relationships		(173)
Deferred acquisition costs, fair value of net assets acquired, before settlement of pre-existing relationships		1
Deferred acquisition costs, net effect of settlement of pre-existing relationships		(1)
Salvage and subrogation recoverable		23
Credit derivative assets		1
Deferred tax assets, net, fair value of net assets acquired, before settlement of pre-existing relationships		194
Deferred tax asset, net effect of settlement of pre-exiting relationships		34
Deferred tax asset, net effect of CIFG acquisition		228
Other assets, fair value of net assets acquired, before settlement of pre-existing relationships		4
Total assets, fair value of net assets acquired, before settlement of pre-existing relationships		1,192
Total assets, net of settlement of pre-existing relationships		(140)
Total assets, Net effect of MBIA UK Acquisition		1,052
Liabilities assumed:
Unearned premium reserves, fair value of net assets acquired, before settlement of pre-existing relationships		306
Unearned premium reserves, net of settlement of pre-existing relationships		(10)
Unearned premium reserves, Net effect of MBIA UK Acquisition		296
Loss and loss adjustments expense reserve, fair value of net assets acquired before settlements of pre-existing relationships		1
Loss and loss adjustments expense reserve, net effect of settlement of pre-existing relationships		(66)
Loss and loss adjustments expense reserve, net effect of CIFG acquisition		(65)
Credit derivative liabilities, fair value of net assets acquired before settlement of pre-existing relationships		68
Other liabilities, fair value of net assets acquired, before settlement of pre-existing relationships		17
Total liabilities, fair value of net assets acquired, before settlement of pre-existing relationships		392
Total liabilities, net of settlement of pre-existing relationships		(76)
Total liabilities, Net effect of MBIA UK Acquisition		316
Net assets effect of MBIA UK Acquisition, fair value of net assets acquired, before settlement of pre-existing relationships		800
Net assets effect of MBIA UK Acquisition, net of settlement of pre-existing relationships		(64)
Net asset effect of MBIA UK Acquisition, Net effect of MBIA UK Acquisition		736
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		357
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, net of settlement of pre-existing relationships		(64)
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, Net effect of MBIA UK Acquisition		293
Deferred tax, net of settlement of pre-existing relationships		(34)
Deferred tax, Net effect of MBIA UK Acquisition		(34)
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		357
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, net of settlement of pre-existing relationships		(98)
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, Net effect of MBIA UK Acquisition		259
Assumed assets including pre-existing relationship		627
Settlement of pre-existing relationship		(173)
Consideration transferred		$ 270
MBIA UK [Member]
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, fair value of net assets acquired, before settlement of pre-existing relationships	16
Other assets, net of settlement of pre-existing relationships	(6)
Other assets, Net effect of MBIA UK Acquisition	10
Total assets, fair value of net assets acquired, before settlement of pre-existing relationships	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, fair value of net assets acquired, before settlement of pre-existing relationships	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 effect of MBIA UK Acquisition, fair value of net assets acquired, before settlement of pre-existing relationships	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
Consideration transferred	$ 329