Shareholders' equity - Narrative (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Apr. 30, 2020	May 04, 2010
Stockholders Equity Note [Line Items]
Common stock, authorized (in shares)	800,000,000	800,000,000	800,000,000
Preferred stock, authorized (in shares)	2,000,000	2,000,000	2,000,000
Common stock, outstanding (in shares)	219,500,000	219,500,000	217,500,000
Treasury stock (in shares)	104,918,360	104,918,360	106,955,082
Preferred stock, issued (in shares)	0	0	0
Preferred stock, outstanding (in shares)	0	0	0
Shares approved to be repurchased under share repurchase program, amount	$ 300,000,000.0	$ 300,000,000.0
Share repurchase program, period in force	3 years
Shares repurchased under share repurchase program (in shares)		0	0	500,000
Cost of common shares repurchased		$ 0	$ 0	$ 5,831,000
Stock Options | Employee
Stockholders Equity Note [Line Items]
Shares reserved for issuance (in shares)					20,000,000.0	60,000,000.0
Restricted Stock Units (RSUs) | Employee
Stockholders Equity Note [Line Items]
Number of shares of common stock received for each RSU granted	1	1
Award vesting percentage		25.00%
Performance Shares
Stockholders Equity Note [Line Items]
Unrecognized compensation cost related to non-vested share-based compensation			$ 0
Performance Shares | Minimum
Stockholders Equity Note [Line Items]
Number of shares ultimately issued for each award (as a percent)		0.00%
Performance Shares | Maximum
Stockholders Equity Note [Line Items]
Number of shares ultimately issued for each award (as a percent)		200.00%
Performance Shares | Employee
Stockholders Equity Note [Line Items]
Number of shares of common stock received for each RSU granted	1	1
Restricted Stock and Restricted Stock Units
Stockholders Equity Note [Line Items]
Unrecognized compensation cost related to non-vested share-based compensation	$ 22,200,000	$ 22,200,000
Unrecognized compensation cost related to non-vested share-based compensation for options, recognition period		2 years 6 months