Fair Value Measurement (Assets Measured at Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets:
Estimated Fair Value	$ 3,734,655	$ 2,457,531
Fixed maturities, trading	45,957	48,456
Equity investments	234,211	120,101
Level 3
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities	24,000	0
U.S. Treasury obligations
Assets:
Estimated Fair Value	245,484	107,059
U.S. Government-sponsored enterprise obligations
Assets:
Estimated Fair Value	17,766	12,261
State and municipal bonds
Assets:
Estimated Fair Value	513,001	332,920
Corporate debt, limited observable inputs | Level 3
Assets:
Estimated Fair Value	15,065	3,265
Residential mortgage-backed securities
Assets:
Estimated Fair Value	482,255	276,541
Agency commercial mortgage-backed securities
Assets:
Estimated Fair Value	15,759	13,310
Other commercial mortgage-backed securities
Assets:
Estimated Fair Value	174,241	113,092
Other asset-backed securities
Assets:
Estimated Fair Value	388,759	273,006
Other investments | Level 3
Assets:
Estimated Fair Value	2,222	0
Recurring
Assets:
Total assets categorized within the fair value hierarchy	4,723,947	3,242,057
Recurring | Level 1
Assets:
Fixed maturities, trading	0	0
Other assets	0	0
Total assets categorized within the fair value hierarchy	534,350	416,757
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities	0
Total liabilities categorized within the fair value hierarchy	0
Recurring | Level 2
Assets:
Fixed maturities, trading	45,957	48,456
Other assets	653	329
Total assets categorized within the fair value hierarchy	3,911,910	2,567,083
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities	0
Total liabilities categorized within the fair value hierarchy	0
Recurring | Level 3
Assets:
Fixed maturities, trading	0	0
Other assets	0	0
Total assets categorized within the fair value hierarchy	27,996	11,958
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities	24,000
Total liabilities categorized within the fair value hierarchy	24,000
Recurring | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Fixed maturities, trading	45,957	48,456
Other assets	653	329
Total assets categorized within the fair value hierarchy	4,474,256	2,995,798
Liabilities, Fair Value Disclosure [Abstract]
Other liabilities	24,000
Total liabilities categorized within the fair value hierarchy	24,000
Recurring | U.S. Treasury obligations | Level 1
Assets:
Estimated Fair Value	0	0
Recurring | U.S. Treasury obligations | Level 2
Assets:
Estimated Fair Value	245,484	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	245,484	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	17,766	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	17,766	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	513,001	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	513,001	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	1,882,325	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	1,882,325	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	15,065	3,265
Recurring | Corporate debt, limited observable inputs | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	15,065	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	482,255	274,509
Recurring | Residential mortgage-backed securities | Level 3
Assets:
Estimated Fair Value	0	2,032
Recurring | Residential mortgage-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	482,255	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	15,759	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	15,759	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	174,241	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	174,241	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	378,902	266,345
Recurring | Other asset-backed securities | Level 3
Assets:
Estimated Fair Value	9,857	6,661
Recurring | Other asset-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Estimated Fair Value	388,759	273,006
Recurring | Financial | Level 1
Assets:
Equity investments	0	13,810
Recurring | Financial | Level 2
Assets:
Equity investments	0	0
Recurring | Financial | Level 3
Assets:
Equity investments	850	0
Recurring | Financial | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	850	13,810
Recurring | Utilities/Energy | Level 1
Assets:
Equity investments	0	564
Recurring | Utilities/Energy | Level 2
Assets:
Equity investments	78	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	78	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	212,528	69,475
Recurring | Bond funds | Level 2
Assets:
Equity investments	0	0
Recurring | Bond funds | Level 3
Assets:
Equity investments	2	0
Recurring | Bond funds | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Equity investments	212,530	69,475
Recurring | All other | Level 1
Assets:
Equity investments	20,753	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	20,753	20,202
Recurring | Short-term investments | Level 1
Assets:
Short-term and other investments	299,240	307,695
Recurring | Short-term investments | Level 2
Assets:
Short-term and other investments	52,645	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	351,885	337,813
Recurring | Other investments | Level 1
Assets:
Short-term and other investments	1,829	1,509
Recurring | Other investments | Level 2
Assets:
Short-term and other investments	102,844	42,607
Recurring | Other investments | Level 3
Assets:
Short-term and other investments	2,222	0
Recurring | Other investments | Fair Value, Inputs, Level 1, 2 and 3
Assets:
Short-term and other investments	106,895	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	$ 249,691	$ 233,711