Schedule II Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows - operating activities
Interest expense paid	$ (11,555)	$ (11,754)	$ (13,521)
Income taxes paid	(16,259)	(27,847)	(24,861)
Other, net, including settlement of payables to subsidiaries	(6,534)	(16,297)	(5,430)
Net cash provided by operating activities	256,586	211,433	207,714
Cash flows - investing activities
Net Cash Provided by (Used in) Investing Activities	(228,664)	(325,360)	(415,849)
Cash flows - financing activities
Dividends paid to shareholders	(46,114)	(44,310)	(42,523)
Proceeds from issuance of Senior Notes due 2025	0	0	246,937
Redemption of Senior Notes due 2016	0	0	(127,292)
Maturity of Senior Notes due 2015	0	0	(75,000)
Principal repayment on Bank Credit Facility	0	0	(38,000)
Proceeds from exercise of stock options	4,190	3,329	1,629
Withholding tax payments on RSUs tendered	(3,245)	(4,015)	(671)
Net cash used in financing activities	(36,965)	115,088	211,969
Net increase (decrease) in cash	(9,043)	1,161	3,834
Cash at beginning of period	16,670	15,509	11,675
Cash at end of period	7,627	16,670	15,509
Parent Company [Member]
Cash flows - operating activities
Interest expense paid	(11,503)	(11,754)	(13,521)
Income taxes paid	(373)	8,914	8,413
Cash dividends received from subsidiaries	56,900	59,600	50,000
Other, net, including settlement of payables to subsidiaries	4,201	581	(3,426)
Net cash provided by operating activities	49,225	57,341	41,466
Cash flows - investing activities
Net increase (decrease) in investments	(2,338)	9,161	15,402
Net Cash Provided by (Used in) Investing Activities	(2,338)	9,161	15,402
Cash flows - financing activities
Dividends paid to shareholders	(46,114)	(44,310)	(42,523)
Proceeds from issuance of Senior Notes due 2025	0	0	246,937
Redemption of Senior Notes due 2016	0	0	(127,292)
Maturity of Senior Notes due 2015	0	0	(75,000)
Principal repayment on Bank Credit Facility	0	0	(38,000)
Acquisition of treasury stock	(1,660)	(21,513)	(21,950)
Proceeds from exercise of stock options	4,190	3,329	1,629
Withholding tax payments on RSUs tendered	(3,245)	(4,015)	(671)
Net cash used in financing activities	(46,829)	(66,509)	(56,870)
Net increase (decrease) in cash	58	(7)	(2)
Cash at beginning of period	68	75	77
Cash at end of period	$ 126	$ 68	$ 75