Acquisitions - Assets and Liabilities Assumed (Details) - MBIA UK Insurance Limited [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	274
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions, Premiums, Net of Commissions Payable	270
Business Combination, Separately Recognized Transactions, Unearned Premium Recognized	(4)
Other assets	16
Business Combination, Separately Recognized Transactions, Other Assets Recognized	(6)
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions , Other Assets	10
Total assets, fair value of net assets acquired	821
Total assets, settlement of pre-existing relationships	(10)
Total assets	811
Liabilities assumed:
Unearned premium reserves, fair value of net liabilities assumed	389
Unearned premium reserves, settlement of pre-existing relationship	(6)
Unearned premium reserves	383
Business Combination, Recognized Identifiable Assets Acquired And Liabilities, Taxes Payable, Current	25
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions, Taxes Payable, Current	25
Other liabilities	4
Business Combination, Separately Recognized Transactions, Other Liabilities Recognized	(5)
Business Combination, Separately Recognized Transactions, Taxes Payable, Current	0
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions , Other Liabilities	(1)
Total liabilities, fair value of net assets acquired	418
Total liabilities, settlement of pre-existing relationships	(11)
Total liabilities	407
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net, Excluding Cash Acquired	403
Business Combination, Separately Recognized Transactions, Net, Excluding Cash Acquired	1
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions, Net, Excluding Cash Acquired	404
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net, Cash Acquired	23
Business Combination, Separately Recognized Transactions, Net, Cash Acquired	0
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions, Net, Cash Acquired	23
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities	(36)
Business Combination, Separately Recognized Transactions, Net, Deferred Tax Liability	0
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Including Separately Recognized Transactions, Deferred Tax Liability	(36)
Net assets resulting from acquisition, fair value of net assets acquired	390
Net assets resulting from acquisition, settlement of pre-existing relationships	1
Net asset effect	391
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after tax, fair value of net assets acquired	56
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax, settlement of pre-existing relationships	1
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, after-tax	57
Deferred tax, settlement of pre-existing relationships	1
Deferred tax	1
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisitions, pre-tax, fair value of net assets acquired	56
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax, settlement of pre-existing relationships	2
Bargain purchase gain and settlement of pre-existing relationships resulting from MBIA UK Acquisition, pre-tax	58
Assumed assets including pre-existing relationship	385
Settlement of pre-existing relationship	5
Consideration transferred	$ 329