Segment Information - Additional Significant Financial Information by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
DAC amortization expense	$ 102,185	$ 96,732	$ 98,919
Income tax expense (benefit)	(80,772)	30,467	35,970
Property and Casualty
Segment Reporting Information [Line Items]
DAC amortization expense	76,967	74,950	73,173
Retirement
Segment Reporting Information [Line Items]
DAC amortization expense	17,759	14,635	18,155
Life
Segment Reporting Information [Line Items]
DAC amortization expense	7,459	7,147	7,591
Operating Segments | Property and Casualty
Segment Reporting Information [Line Items]
Income tax expense (benefit)	(3,279)	4,627	11,274
Operating Segments | Retirement
Segment Reporting Information [Line Items]
Income tax expense (benefit)	(19,498)	20,334	19,873
Operating Segments | Life
Segment Reporting Information [Line Items]
Income tax expense (benefit)	(51,876)	9,775	7,951
Corporate and Other | Corporate and Other
Segment Reporting Information [Line Items]
Income tax expense (benefit)	$ (6,119)	$ (4,269)	$ (3,128)