Fair Value Measurement (Assets Measured at Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets:
Estimated Fair Value	$ 2,563,273	$ 2,457,531
Fixed maturities, trading	45,151	48,456
Equity investments	77,537	120,101
U.S. Treasury obligations
Assets:
Estimated Fair Value	102,877	107,059
U.S. Government-sponsored enterprise obligations
Assets:
Estimated Fair Value	11,944	12,261
State and municipal bonds
Assets:
Estimated Fair Value	322,465	332,920
Corporate debt, limited observable inputs | Level 3
Assets:
Estimated Fair Value	7,769	3,265
Residential mortgage-backed securities
Assets:
Estimated Fair Value	271,548	276,541
Agency commercial mortgage-backed securities
Assets:
Estimated Fair Value	13,718	13,310
Other commercial mortgage-backed securities
Assets:
Estimated Fair Value	121,319	113,092
Other asset-backed securities
Assets:
Estimated Fair Value	290,536	273,006
Other investments | Level 3
Assets:
Estimated Fair Value	2,152	0
Recurring
Assets:
Total assets categorized within the fair value hierarchy	3,263,939	3,242,057
Recurring | Level 1
Assets:
Fixed maturities, trading	0	0
Other assets	0	0
Total assets categorized within the fair value hierarchy	336,641	416,757
Recurring | Level 2
Assets:
Fixed maturities, trading	45,151	48,456
Other assets	836	329
Total assets categorized within the fair value hierarchy	2,681,166	2,567,083
Recurring | Level 3
Assets:
Fixed maturities, trading	0	0
Other assets	0	0
Total assets categorized within the fair value hierarchy	19,234	11,958
Recurring | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Fixed maturities, trading	45,151	48,456
Other assets	836	329
Total assets categorized within the fair value hierarchy	3,037,041	2,995,798
Recurring | U.S. Treasury obligations | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | U.S. Treasury obligations | Level 2
Assets:
Estimated Fair Value	102,877	107,059
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	102,877	107,059
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	11,944	12,261
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	11,944	12,261
Recurring | State and municipal bonds | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | State and municipal bonds | Level 2
Assets:
Estimated Fair Value	322,465	332,920
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	322,465	332,920
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	1,421,097	1,326,077
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	1,421,097	1,326,077
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	7,769	3,265
Recurring | Corporate debt, limited observable inputs | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	7,769	3,265
Recurring | Residential mortgage-backed securities | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Residential mortgage-backed securities | Level 2
Assets:
Estimated Fair Value	270,059	274,509
Recurring | Residential mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	1,489	2,032
Recurring | Residential mortgage-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	271,548	276,541
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	13,718	13,310
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	13,718	13,310
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	121,319	113,092
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	121,319	113,092
Recurring | Other asset-backed securities | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | Other asset-backed securities | Level 2
Assets:
Estimated Fair Value	282,712	266,345
Recurring | Other asset-backed securities | Level 3
Assets:
Estimated Fair Value	7,824	6,661
Recurring | Other asset-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	290,536	273,006
Recurring | Financial | Level 1
Assets:
Equity investments		13,810
Recurring | Financial | Level 2
Assets:
Equity investments		0
Recurring | Financial | Level 3
Assets:
Equity investments		0
Recurring | Financial | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments		13,810
Recurring | Utilities/Energy | Level 1
Assets:
Equity investments		564
Recurring | Utilities/Energy | Level 2
Assets:
Equity investments		0
Recurring | Utilities/Energy | Level 3
Assets:
Equity investments		0
Recurring | Utilities/Energy | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments		564
Recurring | Consumer oriented | Level 1
Assets:
Equity investments		1,262
Recurring | Consumer oriented | Level 2
Assets:
Equity investments		0
Recurring | Consumer oriented | Level 3
Assets:
Equity investments		0
Recurring | Consumer oriented | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments		1,262
Recurring | Industrial | Level 1
Assets:
Equity investments		2,240
Recurring | Industrial | Level 2
Assets:
Equity investments		0
Recurring | Industrial | Level 3
Assets:
Equity investments		0
Recurring | Industrial | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments		2,240
Recurring | Bond funds | Level 1
Assets:
Equity investments	60,264	69,475
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	60,264	69,475
Recurring | All other | Level 1
Assets:
Equity investments	17,273	20,202
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	17,273	20,202
Recurring | Short-term investments | Level 1
Assets:
Short-term and other investments	259,075	307,695
Recurring | Short-term investments | Level 2
Assets:
Short-term and other investments	21,918	30,118
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	280,993	337,813
Recurring | Other investments | Level 1
Assets:
Short-term and other investments	29	1,509
Recurring | Other investments | Level 2
Assets:
Short-term and other investments	67,070	42,607
Recurring | Other investments | Level 3
Assets:
Short-term and other investments	2,152	0
Recurring | Other investments | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Short-term and other investments	69,251	44,116
Recurring | Equity investments | Fair Value Measured at Net Asset Value Per Share | Equity investments
Assets:
Total assets categorized within the fair value hierarchy		12,548
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	$ 226,898	$ 233,711