Income Taxes - Reconciliation of Effective Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Expected federal tax on income	$ 31,041	$ 39,981	$ 45,308
Add (deduct) tax effects of:
Tax-exempt interest	(5,335)	(5,789)	(6,678)
Dividend received deduction	(4,448)	(3,985)	(3,564)
Tax Act DTL re-measurement	(98,988)	0	0
Employee share-based compensation	(3,258)	127	265
Other, net	216	133	639
Total income tax expense	$ (80,772)	$ 30,467	$ 35,970