Goodwill & Other Intangible Assets - Other Intangible Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amortized Intangible Assets:
Finite-Lived Intangible Assets, Gross	$ 1,274,000,000	$ 775,000,000
Accumulated Amortization	(299,000,000)	(132,000,000)
Net Carrying Amount	975,000,000	643,000,000
Total Indefinite Life Intangible Assets [5]	95,000,000	14,000,000
Total Other Intangible Assets	1,369,000,000	789,000,000
Total Other Intangible Assets	1,070,000,000	657,000,000
Value of in-force contracts [1]
Amortized Intangible Assets:
Finite-Lived Intangible Assets, Gross	203,000,000	23,000,000
Accumulated Amortization	(125,000,000)	(23,000,000)
Net Carrying Amount	78,000,000	0
Value of in-force contracts [1] | The Navigators Group, Inc. [Member]
Amortized Intangible Assets:
Finite-lived Intangible Assets Acquired	$ 180
Finite-Lived Intangible Asset, Useful Life	3 years
Customer relationships [2]
Amortized Intangible Assets:
Finite-Lived Intangible Assets, Gross	$ 636,000,000	636,000,000
Accumulated Amortization	(92,000,000)	(49,000,000)
Net Carrying Amount	544,000,000	587,000,000
Customer relationships [2] | The Navigators Group, Inc. [Member]
Amortized Intangible Assets:
Accumulated Amortization	102,000,000	0	$ 0
Customer relationships [2] | Foremost [Domain]
Amortized Intangible Assets:
Finite-lived Intangible Assets Acquired		$ 46,000,000
Finite-Lived Intangible Asset, Useful Life		10 years
Marketing agreement with Aetna
Amortized Intangible Assets:
Finite-Lived Intangible Assets, Gross	16,000,000	$ 16,000,000
Accumulated Amortization	(2,000,000)	(1,000,000)
Net Carrying Amount	14,000,000	15,000,000
Distribution Agreement
Amortized Intangible Assets:
Finite-Lived Intangible Assets, Gross	79,000,000	79,000,000
Accumulated Amortization	(61,000,000)	(56,000,000)
Net Carrying Amount	18,000,000	23,000,000
Distribution and Agency relationships & Other [3] [4]
Amortized Intangible Assets:
Finite-Lived Intangible Assets, Gross	340,000,000	21,000,000
Accumulated Amortization	(19,000,000)	(3,000,000)
Net Carrying Amount	321,000,000	18,000,000
Distribution and Agency relationships & Other [3] [4] | Y-Risk [Domain]
Amortized Intangible Assets:
Finite-lived Intangible Assets Acquired		$ 12,000,000
Finite-Lived Intangible Asset, Useful Life		15 years
Distribution relationships [Member] | The Navigators Group, Inc. [Member]
Amortized Intangible Assets:
Finite-lived Intangible Assets Acquired	$ 302
Finite-Lived Intangible Asset, Useful Life	15 years
Trade Names [Member] | The Navigators Group, Inc. [Member]
Amortized Intangible Assets:
Finite-lived Intangible Assets Acquired	$ 17
Finite-Lived Intangible Asset, Useful Life	10 years
License [Member] | The Navigators Group, Inc. [Member]
Amortized Intangible Assets:
Indefinite-lived Intangible Assets Acquired	$ 15
Lloyd's Syndicate Capacity [Member] | The Navigators Group, Inc. [Member]
Amortized Intangible Assets:
Indefinite-lived Intangible Assets Acquired	$ 66