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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 169,459	$ 83,765	$ 93,482
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized investment (gains) losses													3,406	(4,123)	(12,713)
Increase in accrued investment income													(3,404)	(2,208)	(2,566)
Increase (decrease) in accrued expenses													(2,240)	4,378	(5,798)
Depreciation and amortization													6,615	6,896	7,734
Increase in insurance liabilities													154,061	176,315	145,313
Increase in premium receivables													(12,917)	(11,496)	(8,641)
Increase in deferred policy acquisition costs													(7,967)	(15,859)	(8,981)
(Increase) decrease in reinsurance recoverables													11	(481)	(748)
(Decrease) increase in income tax liabilities													(74,487)	(1,293)	8,935
Debt retirement costs													0	0	2,338
Other													24,049	(24,461)	(10,641)
Total adjustments													87,127	127,668	114,232
Net cash provided by operating activities													$ 256,586	$ 211,433	$ 207,714