Fair Value Measurement (Assets and Liabilities Measured at Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Fixed maturities, available for sale	$ 2,153,683	$ 2,280,242
Equity investments	492,159	470,609
Fair Value	297,309	230,889
Investment in unconsolidated subsidiaries
Assets:
Fair Value	277,094	210,759
Level 3 | Corporate debt, limited observable inputs
Assets:
Total assets categorized within the fair value hierarchy	15,097	13,703
Level 3 | Other investments
Assets:
Total assets categorized within the fair value hierarchy	365	409
Recurring
Assets:
Other assets	2,306	1,731
Total assets categorized within the fair value hierarchy	3,014,242	3,216,879
Total assets at fair value	3,311,551	3,447,768
Recurring | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	129,751	133,627
Recurring | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	29,959	20,956
Recurring | State and municipal bonds
Assets:
Fixed maturities, available for sale	329,995	632,243
Recurring | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	1,233,166	1,153,455
Recurring | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	15,097	13,703
Recurring | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	215,420	197,844
Recurring | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	14,887	10,742
Recurring | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	29,195	15,961
Recurring | Other asset-backed securities
Assets:
Fixed maturities, available for sale	156,213	101,711
Recurring | Corporate Debt
Assets:
Fixed maturities, available for sale	4,148
Recurring | Financial
Assets:
Equity investments	74,556	76,051
Recurring | Utilities/Energy
Assets:
Equity investments	46,637	54,388
Recurring | Consumer oriented
Assets:
Equity investments	54,789	54,529
Recurring | Industrial
Assets:
Equity investments	48,859	53,936
Recurring | Bond funds
Assets:
Equity investments	153,256	156,563
Recurring | All other
Assets:
Equity investments	93,847	75,142
Recurring | Short-term investments
Assets:
Short-term and other investments	349,119	432,126
Recurring | Other investments
Assets:
Short-term and other investments	33,042	32,171
Fair Value		20,130
Recurring | Investment in unconsolidated subsidiaries
Assets:
Fair Value		210,759
Recurring | Level 1
Assets:
Other assets	0	0
Total assets categorized within the fair value hierarchy	762,901	877,791
Recurring | Level 1 | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | State and municipal bonds
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	2,334	2,371
Recurring | Level 1 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 1 | Corporate Debt
Assets:
Fixed maturities, available for sale	0
Recurring | Level 1 | Financial
Assets:
Equity investments	74,556	76,051
Recurring | Level 1 | Utilities/Energy
Assets:
Equity investments	46,637	54,388
Recurring | Level 1 | Consumer oriented
Assets:
Equity investments	54,789	54,529
Recurring | Level 1 | Industrial
Assets:
Equity investments	48,859	53,936
Recurring | Level 1 | Bond funds
Assets:
Equity investments	153,256	156,563
Recurring | Level 1 | All other
Assets:
Equity investments	93,847	75,142
Recurring | Level 1 | Short-term investments
Assets:
Short-term and other investments	288,019	404,204
Recurring | Level 1 | Other investments
Assets:
Short-term and other investments	604	607
Recurring | Level 2
Assets:
Other assets	2,306	1,731
Total assets categorized within the fair value hierarchy	2,218,556	2,319,990
Recurring | Level 2 | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	129,751	133,627
Recurring | Level 2 | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	29,959	20,956
Recurring | Level 2 | State and municipal bonds
Assets:
Fixed maturities, available for sale	329,995	632,243
Recurring | Level 2 | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	1,230,832	1,151,084
Recurring | Level 2 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 2 | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	215,420	196,789
Recurring | Level 2 | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	14,887	10,742
Recurring | Level 2 | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	29,195	15,961
Recurring | Level 2 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	138,890	97,780
Recurring | Level 2 | Corporate Debt
Assets:
Fixed maturities, available for sale	4,148
Recurring | Level 2 | Financial
Assets:
Equity investments	0	0
Recurring | Level 2 | Utilities/Energy
Assets:
Equity investments	0	0
Recurring | Level 2 | Consumer oriented
Assets:
Equity investments	0	0
Recurring | Level 2 | Industrial
Assets:
Equity investments	0	0
Recurring | Level 2 | Bond funds
Assets:
Equity investments	0	0
Recurring | Level 2 | All other
Assets:
Equity investments	0	0
Recurring | Level 2 | Short-term investments
Assets:
Short-term and other investments	61,100	27,922
Recurring | Level 2 | Other investments
Assets:
Short-term and other investments	32,073	31,155
Recurring | Level 3
Assets:
Other assets	0	0
Total assets categorized within the fair value hierarchy	32,785	19,098
Recurring | Level 3 | U.S. Treasury obligations
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 3 | U.S. Government-sponsored enterprise obligations
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 3 | State and municipal bonds
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 3 | Corporate debt, multiple observable inputs
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 3 | Corporate debt, limited observable inputs
Assets:
Fixed maturities, available for sale	15,097	13,703
Recurring | Level 3 | Residential mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	1,055
Recurring | Level 3 | Agency commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 3 | Other commercial mortgage-backed securities
Assets:
Fixed maturities, available for sale	0	0
Recurring | Level 3 | Other asset-backed securities
Assets:
Fixed maturities, available for sale	17,323	3,931
Recurring | Level 3 | Corporate Debt
Assets:
Fixed maturities, available for sale	0
Recurring | Level 3 | Financial
Assets:
Equity investments	0	0
Recurring | Level 3 | Utilities/Energy
Assets:
Equity investments	0	0
Recurring | Level 3 | Consumer oriented
Assets:
Equity investments	0	0
Recurring | Level 3 | Industrial
Assets:
Equity investments	0	0
Recurring | Level 3 | Bond funds
Assets:
Equity investments	0	0
Recurring | Level 3 | All other
Assets:
Equity investments	0	0
Recurring | Level 3 | Short-term investments
Assets:
Short-term and other investments	0	0
Recurring | Level 3 | Other investments
Assets:
Short-term and other investments	365	409
Mortgage fund | Other investments
Assets:
Fair Value	0	20,130
Mortgage fund | Equities
Assets:
Fair Value	$ 20,215	$ 0