Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	$ 7,373,552	$ 7,302,950
Amortized Cost/Cost		116,320
Fair Value	7,634,682	7,724,075
Fair Value	131,023	135,466
Federal National Mortgage Association Certificates and Obligations (FNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	386,398	361,955
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	396,957	400,001
Government National Mortgage Association Certificates and Obligations (GNMA)
Unrealized gains and losses on fixed maturities and equity securities
Fair Value	98,056	104,168
Total Fixed Maturity Securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	7,373,552	7,302,950
Unrealized Investment Gains	324,880	443,633
Unrealized Investment Losses	63,750	22,508
Fair Value	7,634,682	7,724,075
OTTI in AOCI	0	0
Mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	699,629	669,297
Unrealized Investment Gains	22,930	30,460
Unrealized Investment Losses	11,601	3,032
Fair Value	710,958	696,725
OTTI in AOCI	0	0
Other, including U.S. Treasury securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	760,390	714,613
Unrealized Investment Gains	18,797	26,311
Unrealized Investment Losses	17,220	5,516
Fair Value	761,967	735,408
OTTI in AOCI	0	0
Municipal bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	1,721,046	1,711,581
Unrealized Investment Gains	149,774	184,107
Unrealized Investment Losses	9,516	2,435
Fair Value	1,861,304	1,893,253
OTTI in AOCI	0	0
Foreign government bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	94,870	96,780
Unrealized Investment Gains	4,307	5,958
Unrealized Investment Losses	136	0
Fair Value	99,041	102,738
OTTI in AOCI	0	0
Corporate bonds
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	2,383,394	2,409,426
Unrealized Investment Gains	112,451	173,862
Unrealized Investment Losses	14,575	4,334
Fair Value	2,481,270	2,578,954
OTTI in AOCI	0	0
Other mortgage-backed securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost	1,714,223	1,701,253
Unrealized Investment Gains	16,621	22,935
Unrealized Investment Losses	10,702	7,191
Fair Value	1,720,142	1,716,997
OTTI in AOCI	$ 0	0
Equity securities
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost/Cost		116,320
Unrealized Investment Gains		19,425
Unrealized Investment Losses		279
Fair Value		135,466
OTTI in AOCI		$ 0

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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