Customer Trust Fund - Gross Unrealized Losses and Related Fair Value of Investment (Detail) $ in Thousands	Sep. 30, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	$ (13,800)
Less than 12 months, Fair Value	718,500
12 months or more, Unrealized Losses	(18,900)
12 months or more, Fair Value	663,700
Total, Unrealized Losses	(32,700)
Total, Fair Value	1,382,200
U.S. Government and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	(8,000)
Less than 12 months, Fair Value	257,900
12 months or more, Unrealized Losses	(11,100)
12 months or more, Fair Value	268,700
Total, Unrealized Losses	(19,100)
Total, Fair Value	526,600
Canadian and Provincial Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	(900)
Less than 12 months, Fair Value	86,900
12 months or more, Unrealized Losses	(1,800)
12 months or more, Fair Value	100,800
Total, Unrealized Losses	(2,700)
Total, Fair Value	187,700
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	(3,700)
Less than 12 months, Fair Value	271,100
12 months or more, Unrealized Losses	(3,100)
12 months or more, Fair Value	145,900
Total, Unrealized Losses	(6,800)
Total, Fair Value	417,000
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	(1,100)
Less than 12 months, Fair Value	97,400
12 months or more, Unrealized Losses	(2,500)
12 months or more, Fair Value	129,800
Total, Unrealized Losses	(3,600)
Total, Fair Value	227,200
Mortgage-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	(50)
Less than 12 months, Fair Value	1,200
12 months or more, Unrealized Losses	(300)
12 months or more, Fair Value	8,000
Total, Unrealized Losses	(300)
Total, Fair Value	9,200
Other Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Unrealized Losses	(100)
Less than 12 months, Fair Value	4,000
12 months or more, Unrealized Losses	(100)
12 months or more, Fair Value	10,500
Total, Unrealized Losses	(200)
Total, Fair Value	$ 14,500