Revenue from Contracts with Customers - Summary of Consolidated Statement of Cash Flows Line Items, which Reflect Adoption of New Revenue Recognition Guidance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Cash flows from operating activities
Net earnings	$ 675.9	$ 516.9	[1]	$ 430.3	[1]
Adjustments to reconcile net earnings to net cash provided by operating activities:
Net change in premiums and fees receivable		(47.7)		(777.2)
Net change in deferred revenue	18.4	0.9	[1]	15.1	[1]
Net change in premiums payable to underwriting enterprises	819.7	166.9	[1]	770.0	[1]
Net change in other current assets	(134.7)	(35.3)	[1]	(45.5)	[1]
Net change in accrued compensation and other current liabilities		69.6		69.8
Net change in deferred income taxes	(216.0)	(219.3)	[1]	(166.1)	[1]
Net change in other noncurrent assets and liabilities	$ (22.0)	(13.2)	[1]	(27.5)	[1]
As Previously Reported [Member]
Cash flows from operating activities
Net earnings		499.2		445.0
Adjustments to reconcile net earnings to net cash provided by operating activities:
Net change in premiums and fees receivable		(220.3)		(242.8)
Net change in premiums payable to underwriting enterprises		334.3		240.2
Net change in other current assets		(48.5)		(55.2)
Net change in accrued compensation and other current liabilities		69.3		69.1
Net change in deferred income taxes		(183.4)		(158.0)
Net change in other noncurrent assets and liabilities		(11.1)		(34.5)
Impact of Adoption of Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Cash flows from operating activities
Net earnings		17.7		(14.7)
Adjustments to reconcile net earnings to net cash provided by operating activities:
Net change in premiums and fees receivable		172.6		(534.4)
Net change in deferred revenue		0.9		15.1
Net change in premiums payable to underwriting enterprises		(167.4)		529.8
Net change in other current assets		13.2		9.7
Net change in accrued compensation and other current liabilities		0.3		0.7
Net change in deferred income taxes		(35.9)		(8.1)
Net change in other noncurrent assets and liabilities		$ (2.1)		$ 7.0

[1] See Note 3 - Revenues from Contracts with Customers for additional information about the restatements related to Topic 606.