Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ 63,104,000	$ 84,836,000	$ 85,045,000
Federal statutory rate (in hundredths)	35.00%	35.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax	$ 4,903,000	$ 7,048,000	$ 7,379,000
State income tax rate (in hundredths)	2.70%	2.90%	3.00%
Noncontrolling interest	$ 12,340,000	$ 1,363,000	$ 0
Non-controlling interest (in hundredths)	6.80%	0.50%	(0.00%)
Federal renewable energy credit	$ (3,400,000)	$ (3,655,000)	$ (3,404,000)
Federal renewable energy credit rate (in hundreths)	(1.90%)	(1.50%)	(1.40%)
Tax compliance and uncertain tax positions	$ (194,000)	$ (8,987,000)	$ (3,902,000)
Tax compliance and uncertain tax positions (in hundredths)	(0.10%)	(3.70%)	(1.60%)
Domestic production activities	$ 0	$ (3,993,000)	$ (666,000)
Domestic production activities (in hundredths)	(0.00%)	(1.60%)	(0.30%)
Other	$ (11,150,000)	$ (13,385,000)	$ (10,158,000)
Other rate (in hundredths)	(6.10%)	(5.50%)	(4.10%)
Total income tax expense	$ 65,603,000	$ 63,227,000	$ 74,294,000
Total income tax expense (benefit) rate (in hundredths)	36.40%	26.10%	30.60%
Temporary differences [Abstract]
Undistributed earnings from foreign operations	$ 2,400,000
Deferred tax liabilities, undistributed foreign earnings	$ 900,000