Acquisitions - Assets and Liabilities Assumed (Details) - CIFG Holding Inc. [Member] $ in Millions	Jul. 01, 2016 USD ($)
Business Acquisition [Line Items]
Cash purchase price	$ 443	[1]
Total purchase price	270
Identifiable assets acquired:
Investments	770
Cash	8
Premiums receivable, net of commissions payable	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 asset, net, fair value of net assets acquired	194
Deferred tax asset, net, settlement of pre-existing relationships	34
Deferred tax asset, net	228
Other assets	4
Total assets, fair value of net assets acquired	1,192
Total assets, settlement of pre-existing relationships	(140)
Total assets	1,052
Liabilities assumed:
Unearned premium reserves, fair value of net liabilities assumed	306
Unearned premium reserves, settlement of pre-existing relationship	(10)
Unearned premium reserves	296
Loss and loss adjustment expense reserve, fair value of liabilities assumed	1
Loss and loss adjustment expense reserve, settlement of pre-existing relationships	(66)
Loss and loss adjustment expense reserve	(65)
Credit derivative liabilities	68
Other liabilities	17
Total liabilities, fair value of net assets acquired	392
Total liabilities, settlement of pre-existing relationships	(76)
Total liabilities	316
Net assets resulting from acquisition, fair value of net assets acquired	800
Net assets resulting from acquisition, settlement of pre-existing relationships	(64)
Net asset effect	736
Bargain purchase gain and settlement of pre-existing relationships resulting from CIFG Acquisition, after tax, fair value of net assets acquired	357
Bargain purchase gain and settlement of pre-existing relationships resulting from CIFG Acquisition, after-tax, settlement of pre-existing relationships	(64)
Bargain purchase gain and settlement of pre-existing relationships resulting from CIFG Acquisition, after-tax	293
Deferred tax, settlement of pre-existing relationships	(34)
Deferred tax	(34)
Bargain purchase gain and settlement of pre-existing relationships resulting from CIFG Acquisitions, pre-tax, fair value of net assets acquired	357
Bargain purchase gain and settlement of pre-existing relationships resulting from CIFG Acquisition, pre-tax, settlement of pre-existing relationships	(98)
Bargain purchase gain and settlement of pre-existing relationships resulting from CIFG Acquisition, pre-tax	259
Assumed assets including pre-existing relationship	627
Settlement of pre-existing relationship	$ 173

[1] The cash purchase price of $443 million was the cash transferred for the acquisition which was allocated as follows: (1) $270 million for the purchase of net assets of $627 million, and (2) the settlement of pre-existing relationships between CIFG and Assured Guaranty at a fair value of $173 million.