Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Consolidated Statements Of Income [Abstract]
Service revenues and sales (Note 2)	$ 1,938,555	$ 1,782,648	$ 1,666,724
Cost of services provided and goods sold (excluding depreciation)	1,321,126	1,227,644	1,150,532
Selling, general and administrative expenses	305,712	271,209	276,652
Depreciation	40,870	38,464	35,488
Amortization	4,335	399	137
Other operating expenses (Note 19)	9,132	1,300	90,880
Total costs and expenses	1,681,175	1,539,016	1,553,689
Income from operations	257,380	243,632	113,035
Interest expense	(4,535)	(4,990)	(4,272)
Other income--net (Note 10)	8,764	958	8,154
Income before income taxes	261,609	239,600	116,917
Income taxes (Note 11)	(41,686)	(34,056)	(18,740)
Net income	$ 219,923	$ 205,544	$ 98,177
Earnings Per Share (Note 15)
Net income	$ 13.77	$ 12.80	$ 6.11
Average number of shares outstanding	15,969	16,059	16,057
Diluted Earnings Per Share (Note 15)
Net income	$ 13.31	$ 12.23	$ 5.86
Average number of shares outstanding	16,527	16,803	16,742