Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Cumulative Effect, Period of Adoption, Adjustment [Member] Retained Earnings [Member]
Balance, beginning of period at Dec. 31, 2018		$ 427		$ 1,489,520	$ (28,434)
Repurchases of common stock		(16)	$ (22,470)	(165,601)
Share-based compensation			22,474
Net issuance of common stock		4	(4)
Net income	$ 272,834			272,834
Dividends				(43,468)
Change in foreign currency translation adjustment,net of income taxes					8,799
Change in net unrealized investment gains (losses) during the period, net of income taxes					73,746
Balance, end of period at Sep. 30, 2019	$ 1,607,811	415		1,553,285	54,111
Dividends declared per share	$ 1.02
Balance, beginning of period at Jun. 30, 2019		420		1,537,535	47,970
Repurchases of common stock		(6)	(4,523)	(66,149)
Share-based compensation			4,524
Net issuance of common stock		1	(1)
Net income	$ 96,223			96,223
Dividends				(14,324)
Change in foreign currency translation adjustment,net of income taxes					(4,009)
Change in net unrealized investment gains (losses) during the period, net of income taxes					10,150
Balance, end of period at Sep. 30, 2019	$ 1,607,811	415		1,553,285	54,111
Dividends declared per share	$ 0.34
Balance, beginning of period at Dec. 31, 2019	$ 1,652,491	412		1,593,281	58,798
Cumulative effect from the adoption of new accounting standards, net at Dec. 31, 2019	1,593,281					$ (1,240)
Repurchases of common stock	(218,100)	(21)	(23,212)	(200,511)
Share-based compensation			23,215
Net issuance of common stock		3	(3)
Net income	286,080			286,080
Dividends				(48,496)
Change in foreign currency translation adjustment,net of income taxes					(9,055)
Change in net unrealized investment gains (losses) during the period, net of income taxes					41,195
Balance, end of period at Sep. 30, 2020	$ 1,720,446	394		1,629,114	90,938
Dividends declared per share	$ 1.20
Balance, beginning of period at Jun. 30, 2020		397		1,569,689	72,594
Repurchases of common stock		(3)	(4,759)	(36,794)
Share-based compensation			$ 4,759
Net income	$ 112,101			112,101
Dividends				(15,882)
Change in foreign currency translation adjustment,net of income taxes					6,312
Change in net unrealized investment gains (losses) during the period, net of income taxes					12,032
Balance, end of period at Sep. 30, 2020	$ 1,720,446	$ 394		$ 1,629,114	$ 90,938
Dividends declared per share	$ 0.40