Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 13,655	$ 26,882	$ 33,647	$ 25,226	$ 29,152	$ 30,298	$ 24,704	$ 22,330	$ 99,410	$ 106,484	$ 72,988
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of share-based compensation									10,288	17,386	12,342
Amortization of original issue discount on debt									149	521	840
Accretion of deferred credit									(149)	(521)	(840)
Net realized (gains) losses on investments									(2,294)	(1,060)	(5,627)
Deferred income taxes									4,724	(693)	1,118
Excess tax benefits from share-based compensation									1,154	(5,310)	(7,006)
Net change in assets and liabilities relating to operating activities:
Income taxes recoverable									1,004	255	2,477
Income taxes payable										3,510	6,239
Other liabilities and accrued expenses									(505)	625	2,774
Net cash provided by (used in) operating activities									121,396	219,271	123,346
Cash flows from investing activities:
Purchases of equity securities, available for sale									(66,688)	(65,038)	(116,541)
Purchase of fixed maturities, available for sale									(320,131)	(178,198)	(106,885)
Proceeds from sales of equity securities, available for sale									60,558	41,456	163,981
Proceeds from sales of fixed maturities, available for sale									86,018	38,379	17,153
Net cash provided by (used in) investing activities									(174,311)	(83,052)	(77,188)
Cash flows from financing activities:
Repayment of debt									(2,136)	(8,470)	(7,471)
Preferred stock dividend									(10)	(10)	(13)
Common stock dividend									(24,192)	(22,287)	(19,240)
Issuance of common stock									119	511	73
Purchase of treasury stock									(8,510)	(18,649)	(29,736)
Sale of treasury stock									2,965
Purchase of preferred stock										(256)
Proceeds received from issuance of contingently redeemable common stock											19,000
Payments related to tax withholding for share-based compensation									(5,451)	(12,141)	(17,655)
Excess tax benefits (shortfall) from share-based compensation									(1,154)	5,310	7,006
Net cash provided by (used in) financing activities									(38,369)	(54,602)	(48,036)
Net increase (decrease) in cash and cash equivalents									(91,284)	81,617	(1,878)
Cash and cash equivalents at beginning of period				197,014				115,397	197,014	115,397	117,275
Cash and cash equivalents at end of period	105,730				197,014				105,730	197,014	115,397
Parent Company [Member]
Cash flows from operating activities:
Net income									99,410	106,484	72,988
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries									(121,847)	(136,817)	(94,924)
Distribution of income from subsidiaries									46,914	58,224	55,805
Depreciation									2	4	2
Amortization of share-based compensation									10,288	17,386	12,342
Amortization of original issue discount on debt									149	521	840
Accretion of deferred credit									(149)	(521)	(840)
Net realized (gains) losses on investments									(667)	(66)	(625)
Deferred income taxes									4,724	(693)	1,118
Excess tax benefits from share-based compensation									1,154	(5,310)	(7,006)
Net change in assets and liabilities relating to operating activities:
Income taxes recoverable									1,004	255	2,477
Income taxes payable										3,510	6,239
Other operating assets and liabilities									(596)	(1,338)	5,752
Other liabilities and accrued expenses									(2,896)
Net cash provided by (used in) operating activities									37,490	41,639	54,168
Cash flows from investing activities:
Capital contributions to subsidiaries											(5,585)
Purchases of equity securities, available for sale									(2,037)	(1,442)	(15,836)
Purchase of fixed maturities, available for sale									(3,000)		(3,000)
Proceeds from sales of equity securities, available for sale									2,456	1,481	26,060
Proceeds from sales of fixed maturities, available for sale									3,229		770
Net cash provided by (used in) investing activities									648	39	2,409
Cash flows from financing activities:
Repayment of debt										(7,000)	(6,000)
Preferred stock dividend									(10)	(10)	(13)
Common stock dividend									(24,192)	(22,287)	(19,240)
Issuance of common stock									119	511	73
Purchase of treasury stock									(8,510)	(18,649)	(29,736)
Sale of treasury stock									2,965
Purchase of preferred stock										(256)
Proceeds received from issuance of contingently redeemable common stock											19,000
Payments related to tax withholding for share-based compensation									(5,451)	(12,141)	(17,655)
Excess tax benefits (shortfall) from share-based compensation									(1,154)	5,310	7,006
Net cash provided by (used in) financing activities									(36,233)	(54,522)	(46,565)
Net increase (decrease) in cash and cash equivalents									1,905	(12,844)	10,012
Cash and cash equivalents at beginning of period				$ 2,046				$ 14,890	2,046	14,890	4,878
Cash and cash equivalents at end of period	$ 3,951				$ 2,046				$ 3,951	$ 2,046	$ 14,890