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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
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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.

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