Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common stock	Paid-in Capital	Accumulated Other Comprehensive Earnings (Loss)	Retained Earnings	Deferred Compensation	Treasury Stock at Cost	Total
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2017	$ 67,079	$ 233,077	$ 157,919	$ 788,522	$ 8,640	$ (401,639)	$ 853,598
Balance (in shares) at Dec. 31, 2017	44,148,355
Increase (Decrease) in Shareholders' Equity
Net earnings				12,216			12,216
Other comprehensive earnings (loss), net of tax			(26,398)				(26,398)
Deferred compensation					(584)	584
Share-based compensation	$ 104	2,617					2,721
Share-based compensation (in shares)	104,771
Dividends and dividend equivalents				(9,290)			(9,290)
Stockholders' Equity Attributable to Parent, Ending Balance at Mar. 31, 2018	$ 67,183	235,694	(6,973)	930,028	8,056	(401,055)	832,933
Balance (in shares) at Mar. 31, 2018	44,253,126
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2017	$ 67,079	233,077	157,919	788,522	8,640	(401,639)	853,598
Balance (in shares) at Dec. 31, 2017	44,148,355
Increase (Decrease) in Shareholders' Equity
Net earnings							84,839
Other comprehensive earnings (loss), net of tax							(41,769)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2018	$ 674	303,399	(22,344)	983,126	7,765	(400,764)	871,856
Balance (in shares) at Sep. 30, 2018	44,484,654
Increase (Decrease) in Shareholders' Equity
Cumulative-effect adjustment from ASU 2016-01 and 2018-02			(138,494)	138,580			86
Stockholders' Equity Attributable to Parent, Beginning Balance at Mar. 31, 2018	$ 67,183	235,694	(6,973)	930,028	8,056	(401,055)	832,933
Balance (in shares) at Mar. 31, 2018	44,253,126
Increase (Decrease) in Shareholders' Equity
Par value conversion from $1.00 per share to $0.01 per share	$ (66,513)	66,513
Net earnings				33,251			33,251
Other comprehensive earnings (loss), net of tax			(7,675)				(7,675)
Deferred compensation					(120)	120
Share-based compensation	$ 3	1,038					1,041
Share-based compensation (in shares)	86,813
Dividends and dividend equivalents				(9,753)			(9,753)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2018	$ 673	303,245	(14,648)	953,526	7,936	(400,935)	849,797
Balance (in shares) at Jun. 30, 2018	44,339,939
Increase (Decrease) in Shareholders' Equity
Net earnings				39,372			39,372
Other comprehensive earnings (loss), net of tax			(7,696)				(7,696)
Deferred compensation					(171)	171
Share-based compensation	$ 1	154					155
Share-based compensation (in shares)	144,715
Dividends and dividend equivalents				(9,772)			(9,772)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2018	$ 674	303,399	(22,344)	983,126	7,765	(400,764)	871,856
Balance (in shares) at Sep. 30, 2018	44,484,654
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2018	$ 674	305,660	(14,572)	908,079	8,354	(401,353)	806,842
Balance (in shares) at Dec. 31, 2018	44,504,043
Increase (Decrease) in Shareholders' Equity
Net earnings				65,473			65,473
Other comprehensive earnings (loss), net of tax			29,301				29,301
Deferred compensation					(1,039)	1,039
Share-based compensation	$ 1	2,891					2,892
Share-based compensation (in shares)	50,213
Dividends and dividend equivalents				(9,803)			(9,803)
Stockholders' Equity Attributable to Parent, Ending Balance at Mar. 31, 2019	$ 675	308,551	14,729	963,749	7,315	(400,314)	894,705
Balance (in shares) at Mar. 31, 2019	44,554,256
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2018	$ 674	305,660	(14,572)	908,079	8,354	(401,353)	806,842
Balance (in shares) at Dec. 31, 2018	44,504,043
Increase (Decrease) in Shareholders' Equity
Net earnings							138,264
Other comprehensive earnings (loss), net of tax							72,506
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2019	$ 678	318,092	57,934	1,015,915	7,788	(400,787)	999,620
Balance (in shares) at Sep. 30, 2019	44,832,088
Stockholders' Equity Attributable to Parent, Beginning Balance at Mar. 31, 2019	$ 675	308,551	14,729	963,749	7,315	(400,314)	894,705
Balance (in shares) at Mar. 31, 2019	44,554,256
Increase (Decrease) in Shareholders' Equity
Net earnings				40,467			40,467
Other comprehensive earnings (loss), net of tax			27,864				27,864
Deferred compensation					215	(215)
Share-based compensation	$ 2	7,215					7,217
Share-based compensation (in shares)	232,941
Dividends and dividend equivalents				(10,305)			(10,305)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2019	$ 677	315,766	42,593	993,911	7,530	(400,529)	959,948
Balance (in shares) at Jun. 30, 2019	44,787,197
Increase (Decrease) in Shareholders' Equity
Net earnings				32,324			32,324
Other comprehensive earnings (loss), net of tax			15,341				15,341
Deferred compensation					258	(258)
Share-based compensation	$ 1	2,326					2,327
Share-based compensation (in shares)	44,891
Dividends and dividend equivalents				(10,320)			(10,320)
Stockholders' Equity Attributable to Parent, Ending Balance at Sep. 30, 2019	$ 678	$ 318,092	$ 57,934	$ 1,015,915	$ 7,788	$ (400,787)	$ 999,620
Balance (in shares) at Sep. 30, 2019	44,832,088