Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Equity, Fair Value Disclosure [Abstract]
Equity securities	$ 470,609	$ 387,274
LP/LLC and investment fund interests carried at NAV which approximates fair value. These interests, reported as a part of Investment in unconsolidated subsidiaries and Other investments, respectively, are not categorized within the fair value hierarchy.	230,889	204,719
Level 3 | Corporate debt, limited observable inputs
Equity, Fair Value Disclosure [Abstract]
Total assets categorized within the fair value hierarchy	13,703	14,810
Level 3 | Other investments
Equity, Fair Value Disclosure [Abstract]
Total assets categorized within the fair value hierarchy	409	3
Fair Value, Measurements, Recurring
Equity, Fair Value Disclosure [Abstract]
Other assets	1,731
Total assets categorized within the fair value hierarchy	3,216,879	3,474,265
LP/LLC and investment fund interests carried at NAV which approximates fair value. These interests, reported as a part of Investment in unconsolidated subsidiaries and Other investments, respectively, are not categorized within the fair value hierarchy.	230,889	204,719
Total assets at fair value	3,447,768	3,678,984
Fair Value, Measurements, Recurring | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	133,627	146,539
Fair Value, Measurements, Recurring | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	20,956	30,235
Fair Value, Measurements, Recurring | State and municipal bonds
Assets:
Fixed maturities, available for sale	632,243	800,463
Fair Value, Measurements, Recurring | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	1,153,455	1,264,181
Fair Value, Measurements, Recurring | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	13,703	14,810
Fair Value, Measurements, Recurring | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	197,844	217,906
Fair Value, Measurements, Recurring | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	10,742	12,783
Fair Value, Measurements, Recurring | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	15,961	19,611
Fair Value, Measurements, Recurring | Other asset-backed securities
Assets:
Fixed maturities, available for sale	101,711	106,878
Fair Value, Measurements, Recurring | Financial
Equity, Fair Value Disclosure [Abstract]
Equity securities	76,051	81,749
Fair Value, Measurements, Recurring | Utilities/Energy
Equity, Fair Value Disclosure [Abstract]
Equity securities	54,388	52,869
Fair Value, Measurements, Recurring | Consumer oriented
Equity, Fair Value Disclosure [Abstract]
Equity securities	54,529	61,284
Fair Value, Measurements, Recurring | Industrial
Equity, Fair Value Disclosure [Abstract]
Equity securities	53,936	54,265
Fair Value, Measurements, Recurring | Bond funds
Equity, Fair Value Disclosure [Abstract]
Equity securities	156,563	90,002
Fair Value, Measurements, Recurring | All other
Equity, Fair Value Disclosure [Abstract]
Equity securities	75,142	47,105
Fair Value, Measurements, Recurring | Short-term investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	432,126	442,084
Fair Value, Measurements, Recurring | Other investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	32,171	31,501
Fair Value, Measurements, Recurring | Level 1
Equity, Fair Value Disclosure [Abstract]
Other assets	0
Total assets categorized within the fair value hierarchy	877,791	799,066
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | State and municipal bonds
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	2,371	2,339
Fair Value, Measurements, Recurring | Level 1 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Financial
Equity, Fair Value Disclosure [Abstract]
Equity securities	76,051	81,749
Fair Value, Measurements, Recurring | Level 1 | Utilities/Energy
Equity, Fair Value Disclosure [Abstract]
Equity securities	54,388	52,869
Fair Value, Measurements, Recurring | Level 1 | Consumer oriented
Equity, Fair Value Disclosure [Abstract]
Equity securities	54,529	61,284
Fair Value, Measurements, Recurring | Level 1 | Industrial
Equity, Fair Value Disclosure [Abstract]
Equity securities	53,936	54,265
Fair Value, Measurements, Recurring | Level 1 | Bond funds
Equity, Fair Value Disclosure [Abstract]
Equity securities	156,563	79,843
Fair Value, Measurements, Recurring | Level 1 | All other
Equity, Fair Value Disclosure [Abstract]
Equity securities	75,142	27,181
Fair Value, Measurements, Recurring | Level 1 | Short-term investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	404,204	437,580
Fair Value, Measurements, Recurring | Level 1 | Other investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	607	1,956
Fair Value, Measurements, Recurring | Level 2
Equity, Fair Value Disclosure [Abstract]
Other assets	1,731
Total assets categorized within the fair value hierarchy	2,319,990	2,657,379
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	133,627	146,539
Fair Value, Measurements, Recurring | Level 2 | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	20,956	30,235
Fair Value, Measurements, Recurring | Level 2 | State and municipal bonds
Assets:
Fixed maturities, available for sale	632,243	800,463
Fair Value, Measurements, Recurring | Level 2 | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	1,151,084	1,261,842
Fair Value, Measurements, Recurring | Level 2 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	196,789	217,906
Fair Value, Measurements, Recurring | Level 2 | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	10,742	12,783
Fair Value, Measurements, Recurring | Level 2 | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	15,961	19,611
Fair Value, Measurements, Recurring | Level 2 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	97,780	103,871
Fair Value, Measurements, Recurring | Level 2 | Financial
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Utilities/Energy
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Consumer oriented
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Industrial
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 2 | Bond funds
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	10,159
Fair Value, Measurements, Recurring | Level 2 | All other
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	19,924
Fair Value, Measurements, Recurring | Level 2 | Short-term investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	27,922	4,504
Fair Value, Measurements, Recurring | Level 2 | Other investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	31,155	29,542
Fair Value, Measurements, Recurring | Level 3
Equity, Fair Value Disclosure [Abstract]
Other assets	0
Total assets categorized within the fair value hierarchy	19,098	17,820
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | State and municipal bonds
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	13,703	14,810
Fair Value, Measurements, Recurring | Level 3 | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	1,055	0
Fair Value, Measurements, Recurring | Level 3 | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	3,931	3,007
Fair Value, Measurements, Recurring | Level 3 | Financial
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Utilities/Energy
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Consumer oriented
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Industrial
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Bond funds
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 3 | All other
Equity, Fair Value Disclosure [Abstract]
Equity securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Short-term investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	0	0
Fair Value, Measurements, Recurring | Level 3 | Other investments
Equity, Fair Value Disclosure [Abstract]
Short-term investments	$ 409	$ 3