Stock plans (Other awards activity) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Restricted share awards
Activities
Beginning balance (in shares)	2,739,446	2,858,711
Granted (in shares)	559,516	706,341
Vested (in shares)	(789,547)	(783,704)
Forfeited (in shares)	(33,070)	(52,642)
Ending balance (in shares)	2,476,345	2,728,706
Weighted average grant date fair value
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 38.25	$ 35.81
Weighted average grant date fair value, granted (in dollars per share)	48.78	43.58
Weighted average grant date fair value, vested (in dollars per share)	37.36	34.40
Weighted average grant date fair value, forfeited (in dollars per share)	40.25	38.03
Weighted average grant date fair value, ending balance (in dollars per share)	$ 40.88	$ 38.19
Restricted share units
Activities
Beginning balance (in shares)	114,337	103,484
Granted (in shares)	21,609	28,057
Vested (in shares)	(23,982)	(19,455)
Issued in lieu of cash dividends (in shares)	2,436	2,337
Forfeited (in shares)	(8,338)	(892)
Ending balance (in shares)	106,062	113,531
Weighted average grant date fair value
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 38.47	$ 36.54
Weighted average grant date fair value, granted (in dollars per share)	48.83	42.91
Weighted average grant date fair value, vested (in dollars per share)	38.18	34.58
Weighted average grant date fair value, issued in lieu of cash dividends (in dollars per share)	39.10	37.21
Weighted average grant date fair value, forfeited (in dollars per share)	44.34	35.42
Weighted average grant date fair value, ending balance (in dollars per share)	$ 40.20	$ 38.47
Performance shares
Activities
Beginning balance (in shares)	172,594	106,369
Granted (in shares)	125,290	81,569
Vested (in shares)	(57,581)	(15,344)
Conversion adjustment (in shares)	45,517	0
Ending balance (in shares)	285,820	172,594
Weighted average grant date fair value
Weighted average grant date fair value, beginning balance (in dollars per share)	$ 40.70	$ 36.03
Weighted average grant date fair value, granted (in dollars per share)	48.75	45.03
Weighted average grant date fair value, vested (in dollars per share)	36.11	31.38
Weighted average grant date fair value, conversion adjustment (in dollars per share)	36.82	0.00
Weighted average grant date fair value, ending balance (in dollars per share)	$ 44.53	$ 40.70