Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	$ 6,874,863	[1]
Total investments, Fair Value	0
Total investments, Amount shown in Balance Sheet	7,403,467
Short-term investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	142,039	[1]
Total investments, Fair Value	0
Total investments, Amount shown in Balance Sheet	142,039
Policy loans [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	145,341	[1]
Total investments, Fair Value	0
Total investments, Amount shown in Balance Sheet	145,341
Derivative instruments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	2,458	[1]
Total investments, Fair Value	0
Total investments, Amount shown in Balance Sheet	2,458
Mortgage loans [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	68	[1]
Total investments, Fair Value	0
Total investments, Amount shown in Balance Sheet	68
Other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	109,816	[1]
Total investments, Fair Value	0
Total investments, Amount shown in Balance Sheet	109,816
Fixed maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	6,375,237	[1]
Total investments, Fair Value	6,893,090
Total investments, Amount shown in Balance Sheet	6,893,090
Fixed maturities [Member] | U.S. Government and federally sponsored agency obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	997,157	[1]
Total investments, Fair Value	1,073,925
Total investments, Amount shown in Balance Sheet	1,073,925
Fixed maturities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	1,462,717	[1]
Total investments, Fair Value	1,647,822
Total investments, Amount shown in Balance Sheet	1,647,822
Fixed maturities [Member] | Foreign government bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	52,552	[1]
Total investments, Fair Value	59,536
Total investments, Amount shown in Balance Sheet	59,536
Fixed maturities [Member] | Public utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	180,920	[1]
Total investments, Fair Value	211,675
Total investments, Amount shown in Balance Sheet	211,675
Fixed maturities [Member] | Other bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	3,681,891	[1]
Total investments, Fair Value	3,900,132
Total investments, Amount shown in Balance Sheet	3,900,132
Equity securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	99,904	[1]
Total investments, Fair Value	110,655
Total investments, Amount shown in Balance Sheet	110,655
Equity securities [Member] | Non-redeemable preferred stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	22,672	[1]
Total investments, Fair Value	22,175
Total investments, Amount shown in Balance Sheet	22,175
Equity securities [Member] | Common stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	57,228	[1]
Total investments, Fair Value	68,416
Total investments, Amount shown in Balance Sheet	68,416
Equity securities [Member] | Closed-end fund [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments, Cost	20,004	[1]
Total investments, Fair Value	20,064
Total investments, Amount shown in Balance Sheet	$ 20,064

[1] Bonds at original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts and impairment in value of specifically identified investments.