Share-based compensation plans - Stock activity - RSUs, PSUs, MSUs(Details) - $ / shares		6 Months Ended
	Feb. 20, 2015	Jun. 30, 2015
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance, shares		367,100
Granted, shares		110,800
Cancelled, shares		26,200
Vested, shares		83,000
Nonvested ending balance, shares		368,700
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested beginning balance (dollars per share)		$ 25.87
Granted (dollars per share)		26.40
Cancelled (dollars per share)		26.11
Vested (dollars per share)		26.73
Nonvested ending balance (dollars per share)		$ 25.83
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance, shares		343,600
Granted, shares		193,800
Cancelled, shares		13,400
Vested, shares		0
Nonvested ending balance, shares		524,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested beginning balance (dollars per share)		$ 24.06
Granted (dollars per share)		29.08
Cancelled (dollars per share)		24.85
Vested (dollars per share)		0.00
Nonvested ending balance (dollars per share)		$ 25.90
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested beginning balance, shares		163,300
Granted, shares		108,600
Cancelled, shares		0
Vested, shares		0
Nonvested ending balance, shares		271,900
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested beginning balance (dollars per share)		$ 25.47
Granted (dollars per share)	$ 30.37	30.37
Cancelled (dollars per share)		0.00
Vested (dollars per share)		0.00
Nonvested ending balance (dollars per share)		$ 27.42