Condensed Consolidated Statements of Changes in Shareholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Common Stock Subscribed, Unissued	Retained Earnings	Accumulated Other Comprehensive Loss	Noncontrolling Interests
Balance at Dec. 31, 2016	$ 730,889	$ 348,771	$ 0	$ 575,380	$ (196,600)	$ 3,338
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	80,678			80,449		229
Other comprehensive income (loss)	15,958				15,730	228
Stock issued	15,219	14,592				627
Stock purchased	(11,816)	(11,424)				(392)
Advance payments	966		966
Dividends declared	(15,910)			(15,910)
Balance at Sep. 30, 2017	815,984	351,939	966	639,919	(180,870)	4,030
Balance at Dec. 31, 2017	761,548	387,687	0	619,916	(250,154)	4,099
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	119,417			119,147		270
Other comprehensive income (loss)	11,784				11,861	(77)
Stock issued	14,984	14,984
Stock purchased	(13,087)	(12,832)				(255)
Advance payments	1,007		1,007
Dividends declared	(17,587)			(17,587)
Balance at Sep. 30, 2018	878,864	$ 389,839	$ 1,007	722,274	$ (238,293)	$ 4,037
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASC 606, net of tax	$ 798			$ 798