Business Acquisitions - Expected Pre-tax Amortization Expense (Details) - The Navigators Group, Inc. [Member] $ in Millions	Sep. 30, 2019 USD ($)
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year	$ 5
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	22
Finite-Lived Intangible Assets, Amortization Expense, Year Two	22
Finite-Lived Intangible Assets, Amortization Expense, Year Three	22
Finite-Lived Intangible Assets, Amortization Expense, Year Four	22
Contractual Rights [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year	38
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	47
Finite-Lived Intangible Assets, Amortization Expense, Year Two	21
Finite-Lived Intangible Assets, Amortization Expense, Year Three	9
Finite-Lived Intangible Assets, Amortization Expense, Year Four	$ 0