Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ 114,179,000	$ 86,468,000	$ 87,308,000
Federal statutory rate	35.00%	35.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax	$ 9,027,000	$ 8,208,000	$ 7,019,000
State income tax rate	2.80%	3.30%	2.80%
Federal renewable energy credit	$ (13,544,000)	$ (3,400,000)	$ (3,655,000)
Federal renewable energy credit rate	(4.20%)	(1.40%)	(1.50%)
Tax compliance and uncertain tax positions	$ (3,028,000)	$ (2,607,000)	$ (8,568,000)
Tax compliance and uncertain tax positions	(0.90%)	(1.00%)	(3.40%)
Domestic production activities	$ (6,251,000)	$ (6,842,000)	$ (3,993,000)
Domestic production activities	(1.90%)	(2.80%)	(1.60%)
Other	$ (7,251,000)	$ (11,163,000)	$ (13,689,000)
Other rate	(2.30%)	(4.50%)	(5.50%)
Total income tax expense	$ 93,132,000	$ 70,664,000	$ 64,422,000
Total income tax expense rate	28.50%	28.60%	25.80%
Temporary differences [Abstract]
Undistributed earnings from foreign operations	$ 2,400,000
Deferred tax liabilities, undistributed foreign earnings	$ 889,000