Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Insurance premiums and contract charges earned	$ 240,392	$ 239,681	$ 208,096	$ 209,785	$ 201,905	$ 206,820	$ 205,610	$ 202,998	$ 204,328	$ 198,935	$ 195,718	$ 195,722	$ 897,954	$ 817,333	$ 794,703
Insurance premiums written and contract deposits	346,242	371,216	311,691	295,394	311,216	338,097	301,722	284,008	300,416	318,355	311,614	296,732
Total revenues	331,376	334,418	451,478	313,213	278,535	311,318	306,257	295,489	302,993	289,817	291,436	287,304	1,430,485	1,191,599	1,171,550
Net income	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 184,443	$ 18,343	$ 169,459
Basic
Net income - basic (in usd per share)	$ 0.79	$ 0.61	$ 2.25	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.49	$ 3.03	$ 0.64	$ 0.05	$ 0.37	$ 4.42	$ 0.44	$ 4.10
Shares of common stock - weighted average basic (in shares)	41,814,000,000	41,785,000,000	41,762,000,000	41,610,000,000	41,596,000,000	41,683,000,000	41,600,000,000	41,497,000,000	41,419,000,000	41,433,000,000	41,368,000,000	41,135,000,000	41,737,876	41,570,492	41,364,546
Diluted
Net income - diluted (in usd per share)	$ 0.78	$ 0.60	$ 2.24	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.48	$ 3.00	$ 0.64	$ 0.05	$ 0.37	$ 4.40	$ 0.44	$ 4.08
Shares of common stock and equivalent shares - weighted average diluted (in shares)	42,093,000,000	42,030,000,000	41,921,000,000	41,785,000,000	41,911,000,000	41,850,000,000	41,735,000,000	41,653,000,000	41,718,000,000	41,575,000,000	41,493,000,000	41,342,000,000	41,948,531	41,894,232	41,564,979
TCJA effect, earnings per share, basic (in usd per share)									$ 2.39
TCJA effect, earnings per share, diluted (in usd per share)									$ 2.37