Schedule III and VI Supplementary Insurance Information Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 257,826	$ 267,580	$ 253,176
Future policy benefits, claims and claim expenses	5,921,484	5,777,857	5,450,562
Unearned premiums	260,539	246,274	232,841
Other policy claims and benefits payable	724,261	708,950	692,652
Premium revenue/ premium earned	794,703	759,146	731,880
Net investment income	373,630	361,186	332,600
Benefits, claims and settlement expenses	780,941	733,026	679,206
Amortization of deferred policy acquisition costs	102,185	96,732	98,919
Other operating expenses	199,737	184,920	172,871
Property and Casualty
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	29,191	27,604	26,685
Future policy benefits, claims and claim expenses	319,182	307,757	301,569
Discount, if any, deducted in previous column	0	0	0
Unearned premiums	258,502	244,005	230,201
Other policy claims and benefits payable	0	0	0
Premium revenue/ premium earned	648,263	620,514	595,958
Net investment income	36,178	38,997	33,461
Benefits, claims and settlement expenses	496,289	464,098	420,311
Claims and claims adjustment expense incurred related to current year	498,989	471,098	432,811
Claims and claims adjustment expense incurred related to prior years	(2,700)	(7,000)	(12,500)
Amortization of deferred policy acquisition costs	76,967	74,950	73,173
Other operating expenses	96,488	90,802	84,785
Paid claims and claim adjustment expenses	481,074	468,778	436,431
Premiums written	662,760	634,319	605,753
Retirement
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	174,661	188,117	178,300
Future policy benefits, claims and claim expenses	4,466,039	4,372,062	4,082,217
Unearned premiums	705	671	734
Other policy claims and benefits payable	720,926	705,603	689,116
Premium revenue/ premium earned	28,003	24,937	25,378
Net investment income	261,994	249,410	228,378
Benefits, claims and settlement expenses	159,385	151,185	141,893
Amortization of deferred policy acquisition costs	17,759	14,635	18,155
Other operating expenses	49,733	40,289	32,555
Life
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	53,974	51,859	48,191
Future policy benefits, claims and claim expenses	1,136,263	1,098,038	1,066,776
Unearned premiums	1,332	1,598	1,906
Other policy claims and benefits payable	3,335	3,347	3,536
Premium revenue/ premium earned	118,437	113,695	110,544
Net investment income	76,195	73,567	71,614
Benefits, claims and settlement expenses	125,267	117,743	117,002
Amortization of deferred policy acquisition costs	7,459	7,147	7,591
Other operating expenses	36,550	36,806	35,470
Other, including consolidating eliminations | Eliminations
Supplementary Insurance Information, by Segment [Line Items]
Net investment income		(788)
Other operating expenses	$ 16,966	$ 17,023	$ 20,061