Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	$ 7,373,911	$ 7,302,950
Amortized Cost/Cost	0	116,320
Fair Value	7,515,318	7,724,075
Fair Value	111,750	135,466
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	7,373,911	7,302,950
Unrealized Gains	249,085	443,633
Unrealized Losses	107,678	22,508
Fair Value	7,515,318	7,724,075
OTTI in AOCI	0	0
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	778,038	669,297
Unrealized Gains	22,724	30,460
Unrealized Losses	13,321	3,032
Fair Value	787,441	696,725
OTTI in AOCI	0	0
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	835,096	714,613
Unrealized Gains	16,127	26,311
Unrealized Losses	17,681	5,516
Fair Value	833,542	735,408
OTTI in AOCI	0	0
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	1,884,313	1,711,581
Unrealized Gains	133,150	184,107
Unrealized Losses	13,494	2,435
Fair Value	2,003,969	1,893,253
OTTI in AOCI	0	0
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	83,343	96,780
Unrealized Gains	2,321	5,958
Unrealized Losses	760	0
Fair Value	84,904	102,738
OTTI in AOCI	0	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	2,054,105	2,409,426
Unrealized Gains	64,296	173,862
Unrealized Losses	38,891	4,334
Fair Value	2,079,510	2,578,954
OTTI in AOCI	0	0
Other mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost	1,739,016	1,701,253
Unrealized Gains	10,467	22,935
Unrealized Losses	23,531	7,191
Fair Value	1,725,952	1,716,997
OTTI in AOCI	0	0
Equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost/Cost		116,320
Unrealized Gains		19,425
Unrealized Losses		279
Fair Value		135,466
OTTI in AOCI		0
FNMA Obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	441,308	361,955
FHLMC Obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	417,308	400,001
GNMA Obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 96,466	$ 104,168

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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