Summary Of Investments Other Than Investments In Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	$ 4,115,618	$ 3,768,141
Fair value	4,312,161	3,768,091
Amounts in the balance sheet	4,312,161	3,768,091
US Government Agencies Debt Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	22,502	25,131
Fair value	22,637	25,003
Amounts in the balance sheet	22,637	25,003
Municipal Bonds [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	2,435,346	2,599,056
Fair value	2,554,208	2,620,132
Amounts in the balance sheet	2,554,208	2,620,132
Collateralized Mortgage Backed Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	62,566	30,640
Fair value	63,003	30,952
Amounts in the balance sheet	63,003	30,952
Corporate Debt Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	233,730	107,479
Fair value	235,565	105,524
Amounts in the balance sheet	235,565	105,524
Collateralized Debt Obligations [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	200,656	169,626
Fair value	199,218	165,789
Amounts in the balance sheet	199,217	165,789
Other Debt Obligations [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	18,476	37,609
Fair value	18,644	37,761
Amounts in the balance sheet	18,645	37,761
Fixed Maturity Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	2,973,276	2,969,541
Fair value	3,093,275	2,985,161
Amounts in the balance sheet	3,093,275	2,985,161
Common Stock [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	498,514	438,504
Fair value	586,367	430,973
Amounts in the balance sheet	586,367	430,973
Nonredeemable Preferred Stock [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	49,442	34,429
Fair value	49,708	31,433
Amounts in the balance sheet	49,708	31,433
Private Equity Funds [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,137	1,481
Fair value	1,203	1,445
Amounts in the balance sheet	1,203	1,445
Private Equity Funds Net Asset Value [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	99,189	69,668
Fair value	87,473	65,780
Amounts in the balance sheet	87,473	65,780
Equity Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	648,282	544,082
Fair value	724,751	529,631
Amounts in the balance sheet	724,751	529,631
Short-term Investments [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	494,060	254,518
Fair value	494,135	253,299
Amounts in the balance sheet	$ 494,135	$ 253,299