Quarterly Results for 2017 and 2016 - Summary of Quarterly Results for the Periods Presented (Detail) - USD ($) $ / shares in Units, $ 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Premiums earned, net	$ 183,708	$ 174,517	$ 169,009	$ 161,559	$ 163,973	$ 159,534	$ 156,461	$ 152,448	$ 688,793	$ 632,416	$ 503,999
Investment income	4,448	3,085	3,223	2,704	3,489	2,304	2,142	1,605	13,460	9,540	5,155
Total revenues	201,312	190,243	185,487	174,874	178,605	172,436	169,802	164,446	751,916	685,289	546,544
Total expenses	142,721	173,644	137,564	127,503	155,878	128,273	114,908	123,347	581,432	522,406	371,521
Net income	$ 36,396	$ 9,964	$ 29,376	$ 31,199	$ 13,655	$ 26,882	$ 33,647	$ 25,226	$ 106,935	$ 99,410	$ 106,484
Basic earnings per share	$ 1.05	$ 0.29	$ 0.84	$ 0.89	$ 0.39	$ 0.77	$ 0.96	$ 0.73	$ 3.07	$ 2.85	$ 3.06
Diluted earnings per share	$ 1.03	$ 0.28	$ 0.82	$ 0.86	$ 0.38	$ 0.75	$ 0.94	$ 0.71	$ 2.99	$ 2.79	$ 2.97

The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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