Employee benefit plans (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0		$ 0	$ 0
Interest cost	4,089,000	4,220,000		8,103,000	8,610,000
Expected return on assets	(5,234,000)	(5,182,000)		(10,263,000)	(10,462,000)
Amortization of prior service credit	0	0		0	0
Amortization of net actuarial loss	1,385,000	1,514,000		3,178,000	3,107,000
Net periodic benefit cost (credit), including amount capitalized	240,000	552,000		1,018,000	1,255,000
Less amount capitalized	73,000	121,000		180,000	202,000
Net periodic benefit cost (credit)	167,000	431,000		838,000	1,053,000
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	306,000	374,000		753,000	824,000
Interest cost	825,000	895,000		1,633,000	1,844,000
Expected return on assets	(1,175,000)	(1,118,000)		(2,320,000)	(2,267,000)
Amortization of prior service credit	(343,000)	(343,000)		(686,000)	(686,000)
Amortization of net actuarial loss	100,000	299,000		436,000	747,000
Net periodic benefit cost (credit), including amount capitalized	(287,000)	107,000		(184,000)	462,000
Less amount capitalized	(114,000)	4,000		(153,000)	38,000
Net periodic benefit cost (credit)	(173,000)	103,000		(31,000)	424,000
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 1,100,000	$ 1,200,000		$ 2,300,000	$ (700,000)
Curtailment gain			$ 3,300,000