Immaterial Restatement of Prior Period Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue	$ 232,537			$ 221,288			$ 537,679	$ 530,779
Operating expenses:
Income taxes	12,194			11,786			13,524	16,950
Total operating expenses				173,959				446,775
Net operating income	51,567			47,329			79,056	84,004
Net income	$ 42,424	$ 16,996	$ (7,640)	$ 36,173	$ 14,805	$ (762)	$ 51,780	$ 50,216
Earnings per share:
Basic (in dollars per share)	$ 0.88			$ 0.75			$ 1.08	$ 1.04
Diluted (in dollars per share)	$ 0.88			$ 0.75			$ 1.08	$ 1.04
Retained earnings	$ 415,326			$ 385,699	358,538	352,747	$ 415,326	$ 385,699	$ 392,053	$ 362,512
Total common stockholders’ equity	757,795	723,164	714,129	723,140	695,257	688,853	757,795	723,140	$ 730,157	699,221	$ 699,221
Operating activities:
Net income	$ 42,424	$ 16,996	$ (7,640)	36,173	14,805	(762)	51,780	50,216
Other changes in noncurrent assets and liabilities							21,602	644
Net cash provided by operating activities							$ 128,343	119,503
Corrections
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue				2,305				6,917
Operating expenses:
Income taxes				524				1,570
Total operating expenses				524				1,570
Net operating income				1,781				5,347
Net income				$ 1,781				$ 5,347
Earnings per share:
Basic (in dollars per share)				$ 0.03				$ 0.11
Diluted (in dollars per share)				$ 0.03				$ 0.11
Retained earnings				$ 11,106	9,325	7,542		$ 11,106		5,759
Total common stockholders’ equity				11,106	9,325	7,542		11,106		5,759
Operating activities:
Net income				1,781				5,347
Other changes in noncurrent assets and liabilities								(5,347)
Net cash provided by operating activities								0
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue				218,983				523,862
Operating expenses:
Income taxes				11,262				15,380
Total operating expenses				173,435				445,205
Net operating income				45,548				78,657
Net income				$ 34,392				$ 44,869
Earnings per share:
Basic (in dollars per share)				$ 0.72				$ 0.93
Diluted (in dollars per share)				$ 0.72				$ 0.93
Retained earnings				$ 374,593	349,213	345,205		$ 374,593		356,753
Total common stockholders’ equity				712,034	$ 685,932	$ 681,311		712,034		$ 693,462
Operating activities:
Net income				$ 34,392				44,869
Other changes in noncurrent assets and liabilities								5,991
Net cash provided by operating activities								$ 119,503