Stock plans (Other awards activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restricted Stock [Member]
Activities
Beginning balance	2,739,446	2,858,711	2,684,745
Granted	564,345	724,357	1,051,348
Vested	(796,716)	(788,758)	(797,446)
Forfeited	(37,093)	(54,864)	(79,936)
Ending balance	2,469,982	2,739,446	2,858,711
Share Based Payment Award Other Than Options Nonvested Weighted Average Grant Date Fair Value [Abstract]
Weighted average grant date fair value, beginning balance	$ 38.25	$ 35.81	$ 33.74
Weighted average grant date fair value, granted	48.83	43.67	37.81
Weighted average grant date fair value, vested	37.40	34.41	31.44
Weighted average grant date fair value, forfeited	41.27	38.14	36.09
Weighted average grant date fair value, ending balance	$ 40.89	$ 38.25	$ 35.81
Restricted share units [Member]
Activities
Beginning balance	114,337	103,484	66,518
Granted	27,609	28,057	53,025
Vested	(23,982)	(19,455)	(18,325)
Issued in lieu of cash dividends	3,182	3,143	2,266
Forfeited	(8,338)	(892)	0
Ending balance	112,808	114,337	103,484
Share Based Payment Award Other Than Options Nonvested Weighted Average Grant Date Fair Value [Abstract]
Weighted average grant date fair value, beginning balance	$ 38.47	$ 36.54	$ 33.74
Weighted average grant date fair value, granted	50.03	42.91	38.10
Weighted average grant date fair value, vested	38.18	34.58	30.71
Weighted average grant date fair value, issued in lieu of cash dividends	39.36	37.53	35.00
Weighted average grant date fair value, forfeited	44.34	35.42	0
Weighted average grant date fair value, ending balance	$ 40.95	$ 38.47	$ 36.54
Performance Shares [Member]
Activities
Beginning balance	172,594	106,369	101,820
Granted	125,290	81,569	52,639
Vested	(57,581)	(15,344)	(32,746)
Conversion adjustment	45,517	0	(15,344)
Ending balance	285,820	172,594	106,369
Share Based Payment Award Other Than Options Nonvested Weighted Average Grant Date Fair Value [Abstract]
Weighted average grant date fair value, beginning balance	$ 40.70	$ 36.03	$ 33.56
Weighted average grant date fair value, granted	48.75	45.03	37.33
Weighted average grant date fair value, vested	36.11	31.38	32.62
Weighted average grant date fair value, conversion adjustment	36.82	0.00	31.38
Weighted average grant date fair value, ending balance	$ 44.53	$ 40.70	$ 36.03