Revenue from Contracts with Customers - Summary of Expected Revenue Related to Performance Obligations (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 457.7
2019 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	349.6
2020 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	46.2
2021 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	27.5
2022 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	9.8
2023 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	5.9
Thereafter [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	18.7
Brokerage [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	284.7
Brokerage [Member] | 2019 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	253.2
Brokerage [Member] | 2020 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	17.7
Brokerage [Member] | 2021 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	12.1
Brokerage [Member] | 2022 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	0.8
Brokerage [Member] | 2023 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	0.5
Brokerage [Member] | Thereafter [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	0.4
Risk Management [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	173.0
Risk Management [Member] | 2019 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	96.4
Risk Management [Member] | 2020 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	28.5
Risk Management [Member] | 2021 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	15.4
Risk Management [Member] | 2022 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	9.0
Risk Management [Member] | 2023 [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	5.4
Risk Management [Member] | Thereafter [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected revenue related to performance obligations	$ 18.3