Separate Accounts - Schedule of Fair Value of Separate Accounts by Major Category of Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Separate account assets	$ 2,572,872	$ 2,287,953
Fixed Income Securities [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Separate account assets	1,095,968	999,435
Equity Securities [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Separate account assets	1,431,158	1,264,270
Cash and Cash Equivalents [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Separate account assets	47,860	30,064
Due To/From Funds [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Separate account assets	(2,220)	(5,941)
Other [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Separate account assets	$ 106	$ 125