Commitments, Contingencies and Other Items (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies [Line Items]
Initial commitments	$ 1,100,000,000
Fulfilled commitments	708,400,000
Unfunded commitments	$ 411,300,000
Operating lease, weighted average lease term	4 years
Operating lease, right-of-use asset	$ 42,800,000
Operating lease, liability	42,900,000
Operating lease, expense	$ 8,500,000
Operating lease, expense		$ 6,100,000
Lessee, finance lease, term of contract	20 years
Lessee, finance lease, renewal term	30 years
Letter of Credit
Commitments and Contingencies [Line Items]
Letters of credit outstanding	$ 1,100,000,000
Letter of Credit | Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Collateral pledged to support letter of credit	37,500,000
Letter of Credit | Capital Support Agreement | Top Layer Reinsurance Ltd.
Commitments and Contingencies [Line Items]
Maximum mandatory capital contribution in the event of loss that reduced capital and surplus below specified level	$ 50,000,000.0
Investments as a Percentage of Shareholders' Equity
Commitments and Contingencies [Line Items]
Concentration risk percentage	10.00%
Other assets
Commitments and Contingencies [Line Items]
Finance lease, right-of-use asset	$ 19,800,000	20,600,000
Other liabilities
Commitments and Contingencies [Line Items]
Finance lease, liability	25,100,000	25,600,000
Other Income
Commitments and Contingencies [Line Items]
Finance lease, interest expense	2,600,000	2,600,000
Finance lease, right-of-use asset, amortization	$ 900,000	$ 900,000