Investments - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	$ 7,302,950	$ 7,152,127
Amortized Cost/Cost	116,320	134,013
Fair Value	7,724,075	7,456,708
Fair Value	135,466	141,649
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	7,302,950	7,152,127
Unrealized Gains	443,633	374,991
Unrealized Losses	22,508	70,410
Fair Value	7,724,075	7,456,708
OTTI in AOCI	0	1,618
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	669,297	412,891
Unrealized Gains	30,460	33,168
Unrealized Losses	3,032	3,640
Fair Value	696,725	442,419
OTTI in AOCI	0	0
Other, including U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	714,613	458,745
Unrealized Gains	26,311	18,518
Unrealized Losses	5,516	10,120
Fair Value	735,408	467,143
OTTI in AOCI	0	0
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	1,711,581	1,648,252
Unrealized Gains	184,107	143,733
Unrealized Losses	2,435	22,588
Fair Value	1,893,253	1,769,397
OTTI in AOCI	0	0
Foreign government bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	96,780	93,864
Unrealized Gains	5,958	5,102
Unrealized Losses	0	297
Fair Value	102,738	98,669
OTTI in AOCI	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	2,409,426	2,672,818
Unrealized Gains	173,862	152,229
Unrealized Losses	4,334	14,826
Fair Value	2,578,954	2,810,221
OTTI in AOCI	0	0
Other mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	1,701,253	1,865,557
Unrealized Gains	22,935	22,241
Unrealized Losses	7,191	18,939
Fair Value	1,716,997	1,868,859
OTTI in AOCI	0	1,618
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost/Cost	116,320	134,013
Unrealized Gains	19,425	13,210
Unrealized Losses	279	5,574
Fair Value	135,466	141,649
OTTI in AOCI	0	0
FNMA Obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	361,955	196,468
FHLMC Obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	400,001	284,050
GNMA Obligations
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 104,168	$ 115,627

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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