Fair Value Measurement (Assets Measured at Fair Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Estimated Fair Value	$ 2,386,085	$ 2,288,785
Fixed maturities, trading	53,081	47,284
Equity investments	129,385	250,552
U.S. Treasury obligations
Assets:
Estimated Fair Value	117,364	110,467
U.S. Government-sponsored enterprise obligations
Assets:
Estimated Fair Value	11,055	17,340
State and municipal bonds
Assets:
Estimated Fair Value	319,092	296,093
Corporate debt, limited observable inputs | Level 3
Assets:
Estimated Fair Value	3,995	5,079
Residential mortgage-backed securities
Assets:
Estimated Fair Value	247,318	208,408
Agency commercial mortgage-backed securities
Assets:
Estimated Fair Value	14,171	8,221
Other commercial mortgage-backed securities
Assets:
Estimated Fair Value	99,076	71,868
Other asset-backed securities
Assets:
Estimated Fair Value	270,828	236,024
Other investments | Level 3
Assets:
Total assets categorized within the fair value hierarchy	0	3,086
Recurring
Assets:
Total assets categorized within the fair value hierarchy	3,217,287	3,233,830
Recurring | Level 1
Assets:
Fixed maturities, trading	0	0
Other assets	0	0
Total assets categorized within the fair value hierarchy	457,937	545,607
Recurring | Level 2
Assets:
Fixed maturities, trading	53,081	47,284
Other assets	341	760
Total assets categorized within the fair value hierarchy	2,485,112	2,384,065
Recurring | Level 3
Assets:
Fixed maturities, trading	0	0
Other assets	0	0
Total assets categorized within the fair value hierarchy	16,949	11,157
Recurring | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Fixed maturities, trading	53,081	47,284
Other assets	341	760
Total assets categorized within the fair value hierarchy	2,959,998	2,940,829
Recurring | U.S. Treasury obligations | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | U.S. Treasury obligations | Level 2
Assets:
Estimated Fair Value	117,364	110,467
Recurring | U.S. Treasury obligations | Level 3
Assets:
Estimated Fair Value	0	0
Recurring | U.S. Treasury obligations | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	117,364	110,467
Recurring | U.S. Government-sponsored enterprise obligations | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | U.S. Government-sponsored enterprise obligations | Level 2
Assets:
Estimated Fair Value	11,055	17,340
Recurring | U.S. Government-sponsored enterprise obligations | Level 3
Assets:
Estimated Fair Value	0	0
Recurring | U.S. Government-sponsored enterprise obligations | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	11,055	17,340
Recurring | State and municipal bonds | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | State and municipal bonds | Level 2
Assets:
Estimated Fair Value	319,092	296,093
Recurring | State and municipal bonds | Level 3
Assets:
Estimated Fair Value	0	0
Recurring | State and municipal bonds | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	319,092	296,093
Recurring | Corporate debt, multiple observable inputs | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Corporate debt, multiple observable inputs | Level 2
Assets:
Estimated Fair Value	1,303,186	1,335,285
Recurring | Corporate debt, multiple observable inputs | Level 3
Assets:
Estimated Fair Value	0	0
Recurring | Corporate debt, multiple observable inputs | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	1,303,186	1,335,285
Recurring | Corporate debt, limited observable inputs | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Corporate debt, limited observable inputs | Level 2
Assets:
Estimated Fair Value	0	0
Recurring | Corporate debt, limited observable inputs | Level 3
Assets:
Estimated Fair Value	3,995	5,079
Recurring | Corporate debt, limited observable inputs | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	3,995	5,079
Recurring | Residential mortgage-backed securities | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Residential mortgage-backed securities | Level 2
Assets:
Estimated Fair Value	243,198	208,408
Recurring | Residential mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	4,120	0
Recurring | Residential mortgage-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	247,318	208,408
Recurring | Agency commercial mortgage-backed securities | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Agency commercial mortgage-backed securities | Level 2
Assets:
Estimated Fair Value	14,171	8,221
Recurring | Agency commercial mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	0	0
Recurring | Agency commercial mortgage-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	14,171	8,221
Recurring | Other commercial mortgage-backed securities | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Other commercial mortgage-backed securities | Level 2
Assets:
Estimated Fair Value	99,076	71,868
Recurring | Other commercial mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	0	0
Recurring | Other commercial mortgage-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	99,076	71,868
Recurring | Other asset-backed securities | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Other asset-backed securities | Level 2
Assets:
Estimated Fair Value	261,994	233,032
Recurring | Other asset-backed securities | Level 3
Assets:
Estimated Fair Value	8,834	2,992
Recurring | Other asset-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	270,828	236,024
Recurring | Financial | Level 1
Assets:
Equity investments	10,622	40,294
Recurring | Financial | Level 2
Assets:
Equity investments	0	0
Recurring | Financial | Level 3
Assets:
Equity investments	0	0
Recurring | Financial | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	10,622	40,294
Recurring | Utilities/Energy | Level 1
Assets:
Equity investments	436	21,195
Recurring | Utilities/Energy | Level 2
Assets:
Equity investments	0	0
Recurring | Utilities/Energy | Level 3
Assets:
Equity investments	0	0
Recurring | Utilities/Energy | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	436	21,195
Recurring | Consumer oriented | Level 1
Assets:
Equity investments	1,144	29,288
Recurring | Consumer oriented | Level 2
Assets:
Equity investments	0	0
Recurring | Consumer oriented | Level 3
Assets:
Equity investments	0	0
Recurring | Consumer oriented | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	1,144	29,288
Recurring | Industrial | Level 1
Assets:
Equity investments	1,707	26,440
Recurring | Industrial | Level 2
Assets:
Equity investments	0	0
Recurring | Industrial | Level 3
Assets:
Equity investments	0	0
Recurring | Industrial | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	1,707	26,440
Recurring | Bond funds | Level 1
Assets:
Equity investments	72,044	58,346
Recurring | Bond funds | Level 2
Assets:
Equity investments	0	0
Recurring | Bond funds | Level 3
Assets:
Equity investments	0	0
Recurring | Bond funds | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	72,044	58,346
Recurring | All other | Level 1
Assets:
Equity investments	19,285	52,512
Recurring | All other | Level 2
Assets:
Equity investments	0	0
Recurring | All other | Level 3
Assets:
Equity investments	0	0
Recurring | All other | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	19,285	52,512
Recurring | Short-term investments | Level 1
Assets:
Short-term and other investments	351,206	317,313
Recurring | Short-term investments | Level 2
Assets:
Short-term and other investments	26,333	22,594
Recurring | Short-term investments | Level 3
Assets:
Short-term and other investments	0	0
Recurring | Short-term investments | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Short-term and other investments	377,539	339,907
Recurring | Other investments | Level 1
Assets:
Short-term and other investments	1,493	219
Recurring | Other investments | Level 2
Assets:
Short-term and other investments	36,221	32,713
Recurring | Other investments | Level 3
Assets:
Short-term and other investments	0	3,086
Recurring | Other investments | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Short-term and other investments	37,714	36,018
Recurring | Equity investments | Fair Value Measured at Net Asset Value Per Share | Equity investments
Assets:
Total assets categorized within the fair value hierarchy	24,147	22,477
Recurring | Equity investments | Fair Value Measured at Net Asset Value Per Share | Investment in unconsolidated subsidiaries
Assets:
Total assets categorized within the fair value hierarchy	$ 233,142	$ 270,524