Employee benefit plans (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0		$ 0	$ 0	$ 86,000
Interest cost	4,305,000		4,285,000	12,915,000	12,855,000
Expected return on assets	(5,231,000)		(5,563,000)	(15,693,000)	(16,689,000)
Amortization of prior service credit	0		0	0	36,000
Amortization of net actuarial loss	1,553,000		1,734,000	4,660,000	5,282,000
Curtailment loss				0	258,000
Net periodic benefit cost, including amount capitalized	627,000		456,000	1,882,000	1,828,000
Less amount capitalized	82,000		90,000	284,000	219,000
Net periodic benefit cost	545,000		366,000	1,598,000	1,609,000
Other postretirement benefits [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	412,000		454,000	1,236,000	1,362,000
Interest cost	922,000		902,000	2,766,000	2,705,000
Expected return on assets	(1,133,000)		(1,199,000)	(3,400,000)	(3,597,000)
Amortization of prior service credit	(343,000)		(343,000)	(1,029,000)	(1,028,000)
Amortization of net actuarial loss	371,000		511,000	1,118,000	1,525,000
Curtailment loss				0	0
Net periodic benefit cost, including amount capitalized	229,000		325,000	691,000	967,000
Less amount capitalized	(34,000)		36,000	4,000	98,000
Net periodic benefit cost	263,000		289,000	687,000	869,000
Supplemental employee retirement plans [Member]
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Curtailment loss		$ (3,300,000)
Net periodic benefit cost	$ 1,300,000		$ 1,700,000	$ 600,000	$ 5,300,000