Schedule II Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands		3 Months Ended												12 Months Ended
	Dec. 07, 2017	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	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income		$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 184,443	$ 18,343	$ 169,459
Changes in:
Other operating assets and liabilities														53,406	(2,048)	(1,692)
Net cash provided by operating activities														127,573	200,888	256,586
Cash flows from investing activities
Acquisition of businesses														(421,516)	0	0
Net cash provided by (used in) investing activities														55,942	(186,499)	(228,664)
Cash flows from financing activities
Dividends paid to shareholders														(47,333)	(46,689)	(46,114)
Principal borrowings on senior revolving credit facility	$ 25,000									50,000				135,000	0	0
Proceeds from exercise of stock options														1,730	3,627	4,190
Withholding tax payments on RSUs tendered														(3,680)	(3,165)	(3,245)
Net cash used in financing activities														(169,913)	(10,110)	(36,965)
Cash at beginning of period					11,906				7,627					11,906	7,627
Cash at end of period		25,206				11,906				7,627				25,206	11,906	7,627
Parent company
Cash flows from operating activities
Net income														184,443	18,343	169,459
Equity in net earnings of subsidiaries														(205,453)	(36,310)	(179,694)
Dividends received from subsidiaries														363,250	61,000	56,900
Changes in:
Income taxes														3,369	(4,939)	(7,041)
Other operating assets and liabilities														8,310	(1,792)	(260)
Other														686	13,804	9,861
Net cash provided by operating activities														354,605	50,106	49,225
Cash flows from investing activities
Net increase (decrease) in short-term investments														3,336	1,621	(2,338)
Acquisition of businesses														(444,124)	0	0
Net cash provided by (used in) investing activities														(440,788)	1,621	(2,338)
Cash flows from financing activities
Dividends paid to shareholders														(47,333)	(46,689)	(46,114)
Principal borrowings on senior revolving credit facility														135,000	0	0
Acquisition of treasury stock														0	(5,088)	(1,660)
Proceeds from exercise of stock options														1,730	3,627	4,190
Withholding tax payments on RSUs tendered														(3,680)	(3,165)	(3,245)
Net cash used in financing activities														85,717	(51,315)	(46,829)
Net increase (decrease) in cash														(466)	412	58
Cash at beginning of period					$ 538				$ 126				$ 68	538	126	68
Cash at end of period		$ 72				$ 538				$ 126				$ 72	$ 538	$ 126