Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
U.S. Treasury obligations [Member]
Fixed maturities, available for sale
Amortized Cost	$ 169,567	$ 191,642
Available for Sale Securities Accumulated Gross Unrealized Gains	7,931	14,266
Available for Sale Securities Accumulated Gross Unrealized Loss	(950)	(51)
Total Fair Value	176,548	205,857
U.S. Government-sponsored enterprise obligations [Member]
Fixed maturities, available for sale
Amortized Cost	34,024	52,110
Available for Sale Securities Accumulated Gross Unrealized Gains	2,662	4,837
Available for Sale Securities Accumulated Gross Unrealized Loss	(312)	0
Total Fair Value	36,374	56,947
State and municipal bonds [Member]
Fixed maturities, available for sale
Amortized Cost	1,142,920	1,134,744
Available for Sale Securities Accumulated Gross Unrealized Gains	51,603	85,329
Available for Sale Securities Accumulated Gross Unrealized Loss	(7,411)	(94)
Total Fair Value	1,187,112	1,219,979
Corporate debt [Member]
Fixed maturities, available for sale
Amortized Cost	1,340,407	1,375,880
Available for Sale Securities Accumulated Gross Unrealized Gains	56,409	96,187
Available for Sale Securities Accumulated Gross Unrealized Loss	(13,583)	(1,543)
Total Fair Value	1,383,233	1,470,524
Residential mortgage-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	239,952	272,990
Available for Sale Securities Accumulated Gross Unrealized Gains	9,667	17,070
Available for Sale Securities Accumulated Gross Unrealized Loss	(1,736)	(210)
Total Fair Value	247,883	289,850
Agency commercial mortgage-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	30,936	57,234
Available for Sale Securities Accumulated Gross Unrealized Gains	494	2,255
Available for Sale Securities Accumulated Gross Unrealized Loss	(64)	(25)
Total Fair Value	31,366	59,464
Other commercial mortgage-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	63,178	69,062
Available for Sale Securities Accumulated Gross Unrealized Gains	3,022	5,049
Available for Sale Securities Accumulated Gross Unrealized Loss	(169)	(5)
Total Fair Value	66,031	74,106
Other asset-backed securities [Member]
Fixed maturities, available for sale
Amortized Cost	83,264	70,670
Available for Sale Securities Accumulated Gross Unrealized Gains	575	1,203
Available for Sale Securities Accumulated Gross Unrealized Loss	(329)	(601)
Total Fair Value	83,510	71,272
Debt Securities [Member]
Fixed maturities, available for sale
Amortized Cost	3,104,248	3,224,332
Available for Sale Securities Accumulated Gross Unrealized Gains	132,363	226,196
Available for Sale Securities Accumulated Gross Unrealized Loss	(24,554)	(2,529)
Total Fair Value	$ 3,212,057	$ 3,447,999