Fair Value Measurement - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	May 05, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill		$ 49,610,000	$ 49,610,000	$ 210,725,000
Goodwill impairment	$ 161,100,000	0	161,115,000	0
Deferred compensation plan assets		39,500,000	30,600,000
Deferred compensation liabilities		52,332,000	30,334,000
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation liabilities		52,300,000	30,300,000
Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, net asset (liability)		0	0
Specialty P&C
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill	161,100,000	0	0	$ 161,115,000
Goodwill impairment	$ 161,100,000	0	$ 161,115,000
NORCAL Group
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration, maximum estimate					$ 84,000,000
Deferred compensation plan assets		5,200,000
NORCAL Group | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation liabilities		$ 18,400,000
Corporate debt, limited observable inputs | Rating BBB
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit quality percentage		100.00%
Corporate debt, limited observable inputs | Rating BBplus
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit quality percentage			100.00%
Other asset-backed securities | BBB Plus
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit quality percentage		100.00%
Other asset-backed securities | Rating AA minus
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit quality percentage			51.00%
Mortgage Loans | Three month LIBOR
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis spread on variable rate for debt		1.325%	1.58%
Interest Rate Cap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid on derivative for right to receive cash payments		$ 2,000,000
Floor interest rate on derivative		2.35%
Interest Rate Cap | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional Amount		$ 35,000,000