Schedule II Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ 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, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
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
Changes in:
Other operating assets and liabilities														(4,539)	53,406	(2,048)
Net cash provided by operating activities														259,794	127,573	200,888
Cash flows from investing activities
Purchase of equity securities														(37,353)	(15,583)	(13,430)
Change in short-term and other investments, net														12,061	(49,325)	(56,192)
Acquisition of businesses														0	(421,516)	0
Net cash (used in) provided by investing activities														(406,757)	55,942	(186,499)
Cash flows from financing activities
Dividends paid to shareholders														(49,620)	(47,333)	(46,689)
Principal borrowings on Bank Credit Facility													$ 50,000
Proceeds from exercise of stock options														2,402	1,730	3,627
Withholding tax payments on RSUs tendered														(2,299)	(3,680)	(3,165)
Net cash provided by (used in) financing activities														143,778	(169,913)	(10,110)
Cash at beginning of period				25,206				11,906						25,206	11,906
Cash at end of period	21,774				25,206				11,906					21,774	25,206	11,906
Parent company
Cash flows from operating activities
Net income														133,315	184,443	18,343
Equity in net earnings of subsidiaries														(148,703)	(205,453)	(36,310)
Dividends received from subsidiaries														167,000	363,250	61,000
Changes in:
Income taxes														1,956	3,369	(4,939)
Other operating assets and liabilities														(5,894)	8,310	(1,792)
Other														1,681	686	13,804
Net cash provided by operating activities														149,355	354,605	50,106
Cash flows from investing activities
Purchase of equity securities														1,000	0	0
Change in short-term and other investments, net														(1,456)	3,336	1,621
Capital contributions to subsidiaries														(97,000)	0	0
Acquisition of businesses														0	(444,124)	0
Net cash (used in) provided by investing activities														(97,456)	(440,788)	1,621
Cash flows from financing activities
Dividends paid to shareholders														(49,620)	(47,333)	(46,689)
Principal borrowings on Bank Credit Facility														0	135,000	0
Acquisition of treasury stock														(2,145)	0	(5,088)
Proceeds from exercise of stock options														2,402	1,730	3,627
Withholding tax payments on RSUs tendered														(2,299)	(3,680)	(3,165)
Net cash provided by (used in) financing activities														(51,662)	85,717	(51,315)
Net increase (decrease) in cash														237	(466)	412
Cash at beginning of period				$ 72				$ 538				$ 126		72	538	126
Cash at end of period	$ 309				$ 72				$ 538				$ 126	$ 309	$ 72	$ 538