Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	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, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Insurance premiums and contract charges earned	$ 233,648	$ 235,353	$ 225,431	$ 236,265	$ 240,392	$ 239,681	$ 208,096	$ 209,785	$ 201,905	$ 206,820	$ 205,610	$ 202,998	$ 930,697	$ 897,954	$ 817,333
Insurance premiums written and contract deposits	332,559	368,853	329,169	328,758	346,242	371,216	311,691	295,394	311,216	338,097	301,722	284,008	1,359,339	1,324,542	1,235,043
Total revenues	352,344	337,080	314,459	306,558	331,376	334,418	451,478	313,213	278,535	311,318	306,257	295,489	1,310,441	1,430,485	1,191,599
Net income	$ 47,791	$ 36,474	$ 30,578	$ 18,472	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 133,315	$ 184,443	$ 18,343
Basic
Net income - basic (in usd per share)	$ 1.14	$ 0.87	$ 0.73	$ 0.44	$ 0.79	$ 0.61	$ 2.25	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.49	$ 3.18	$ 4.42	$ 0.44
Shares of common stock - weighted average basic (in shares)	41,962,000	41,916,000	41,879,000	41,827,000	41,814,000	41,785,000	41,762,000	41,610,000	41,596,000	41,683,000	41,600,000	41,497,000	41,881,289	41,737,876	41,570,492
Diluted
Net income - diluted (in usd per share)	$ 1.13	$ 0.87	$ 0.73	$ 0.44	$ 0.78	$ 0.60	$ 2.24	$ 0.77	$ (0.49)	$ 0.30	$ 0.14	$ 0.48	$ 3.17	$ 4.40	$ 0.44
Shares of common stock and equivalent shares - weighted average diluted (in shares)	42,156,000	42,058,000	41,996,000	42,019,000	42,093,000	42,030,000	41,921,000	41,785,000	41,911,000	41,850,000	41,735,000	41,653,000	42,040,892	41,948,531	41,894,232