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Fair Value Measurement (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	May 05, 2021
Credit Derivatives [Line Items]
Goodwill impairment	$ 0	$ 161,115,000	$ 0	$ 161,115,000	$ 161,115,000
Fair value of funded deferred compensation assets	38,500,000		38,500,000		30,600,000
Deferred compensation liabilities	51,456,000		51,456,000		30,334,000
Fair Value
Credit Derivatives [Line Items]
Deferred compensation liabilities	51,500,000		51,500,000		30,300,000
Fair Value, Nonrecurring
Credit Derivatives [Line Items]
Fair value, net asset (liability)	0		0		0
Specialty P&C
Credit Derivatives [Line Items]
Goodwill impairment		$ 161,100,000	0		$ 161,115,000
NORCAL Group
Credit Derivatives [Line Items]
Contingent consideration, maximum estimate						$ 84,000,000
Fair value of funded deferred compensation assets	5,100,000		5,100,000
NORCAL Group | Fair Value
Credit Derivatives [Line Items]
Deferred compensation liabilities	18,000,000		$ 18,000,000
Corporate debt, limited observable inputs | NRSRO, Rating BBB
Credit Derivatives [Line Items]
Credit quality percentage			100.00%
Corporate debt, limited observable inputs | NRSRO, Rating BB plus
Credit Derivatives [Line Items]
Credit quality percentage					100.00%
Other asset-backed securities | NRSRO, Rating AA Minus
Credit Derivatives [Line Items]
Credit quality percentage					51.00%
Other asset-backed securities | NRSRO, Rating A
Credit Derivatives [Line Items]
Credit quality percentage			95.00%
Mortgages | LIBOR
Credit Derivatives [Line Items]
Basis spread on variable rate for debt			1.325%		1.325%
Interest Rate Cap
Credit Derivatives [Line Items]
Premium paid on derivative for right to receive cash payments	$ 2,000,000		$ 2,000,000
Floor interest rate on derivative	2.35%		2.35%
Interest Rate Cap | Other assets
Credit Derivatives [Line Items]
Derivative asset, notional amount	$ 35,000,000		$ 35,000,000

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