Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Line Items]
Net income from continuing operations	$ 43,418	$ 42,011
Loss from discontinued operations, net of tax	(217)	(474)
Net income	$ 43,201	$ 41,537
Average common shares outstanding:
Average common shares outstanding - basic (in shares)	28,906	28,753
Additional shares for stock-based compensation plans	64	50
Average common shares outstanding - diluted (in shares)	28,970	28,803
Earnings from continuing operations per share of common stock:
Basic (in dollars per share)	$ 1.50	$ 1.46
Diluted (in dollars per share)	1.50	1.46
Earnings (loss) per share of common stock
Basic (in dollars per share)	1.49	1.44
Diluted (in dollars per share)	$ 1.49	$ 1.44
Antidilutive shares	5	16
Discontinued Operations, Held-for-sale [Member] | Gill Ranch [Member]
Income Statement [Line Items]
Loss from discontinued operations, net of tax	$ (217)	$ (474)
Loss from discontinued operations per share of common stock:
Basic (in dollars per share)	$ (0.01)	$ (0.02)
Diluted (in dollars per share)	$ (0.01)	$ (0.02)