Supplementary Data on Cash Flows - Reconciliation of Net Income to Cash Provided by Operating Activities (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 18,343	$ 169,459	$ 83,765
Adjustments to reconcile net income to net cash provided by operating activities:
Net investment (gains) losses													12,543	3,406	(4,123)
Increase in accrued investment income													4,449	(3,404)	(2,208)
Increase (decrease) in accrued expenses													(1,088)	(2,240)	4,378
Depreciation and amortization													7,357	6,615	6,896
Increase in insurance liabilities													197,472	154,061	176,315
Increase in premium receivables													(10,026)	(12,917)	(11,496)
Increase in deferred policy acquisition costs													(783)	(7,967)	(15,859)
(Increase) decrease in reinsurance recoverables													(21,317)	11	(481)
Increase (decrease) in income tax liabilities													5,971	(21,291)	(1,293)
Other													(12,033)	(29,147)	(24,461)
Total adjustments													182,545	87,127	127,668
Net cash provided by operating activities													$ 200,888	$ 256,586	$ 211,433