Pension and other postretirement benefits - Change in Plan Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in fair value of plan assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 276,741	$ 280,862
Fair value of assets at end of year		276,741	$ 280,862
Defined Benefit Pension Plans [Member]
Change in fair value of plan assets [Roll Forward]
Fair value of plan assets at beginning of year	276,741	280,862	305,520
Actual return on plan assets		23,932	(11,321)
Employer contributions	437	1,775	5,910
Medicare Part D subsidy received		0	0
Benefits paid		(29,828)	(19,247)
Fair value of assets at end of year		276,741	280,862
Other Postretirement Benefit Plans [Member]
Change in fair value of plan assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 0	0	0
Actual return on plan assets		0	0
Employer contributions		7,211	5,878
Medicare Part D subsidy received		38	0
Benefits paid		(7,249)	(5,878)
Fair value of assets at end of year		$ 0	$ 0

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)
-URI http://asc.fasb.org/extlink&oid=6504192

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)
-URI http://asc.fasb.org/extlink&oid=6504226

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of contributions made by the employer to defined benefit plans.

+ References

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

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- Definition

Amount of the period's gross receipts received from the prescription drug subsidy, which is to be used in the roll forward of the accumulated postretirement benefit obligation of an applicable postretirement benefit plan.

+ References

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