Employee benefit plans (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0		$ 46,000	$ 0	$ 86,000
Interest cost	4,220,000		4,206,000	8,610,000	8,570,000
Expected return on assets	(5,182,000)		(5,753,000)	(10,462,000)	(11,126,000)
Amortization of prior service cost (credit)	0		18,000	0	36,000
Amortization of net actuarial loss	1,514,000		1,813,000	3,107,000	3,548,000
Curtailment loss	0		258,000	0	258,000
Net periodic benefit cost, including amount capitalized	552,000		588,000	1,255,000	1,372,000
Less amount capitalized	121,000		53,000	202,000	129,000
Net periodic benefit cost	431,000		535,000	1,053,000	1,243,000
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	374,000		425,000	824,000	908,000
Interest cost	895,000		889,000	1,844,000	1,803,000
Expected return on assets	(1,118,000)		(1,223,000)	(2,267,000)	(2,398,000)
Amortization of prior service cost (credit)	(343,000)		(343,000)	(686,000)	(685,000)
Amortization of net actuarial loss	299,000		553,000	747,000	1,014,000
Curtailment loss	0		0	0	0
Net periodic benefit cost, including amount capitalized	107,000		301,000	462,000	642,000
Less amount capitalized	4,000		33,000	38,000	62,000
Net periodic benefit cost	103,000		268,000	424,000	580,000
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Curtailment loss		$ (3,300,000)
Net periodic benefit cost	$ 1,200,000		$ 1,900,000	$ (700,000)	$ 3,600,000