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Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended					12 Months Ended
	Mar. 31, 2017 USD ($) quote / Investment	Dec. 31, 2016 USD ($) quote / Investment	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) quote / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quote / Investment	2.3	2.1				2.1
Quotes per equity investment | quote / Investment	4.0	3.9				3.9
Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued					$ 85.0
Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued				$ 125.0
Aspen Holdings and Third Parties | Silverton 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued		$ 130.0
Third party | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued					$ 70.0
Third party | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued				$ 100.0
Third party | Silverton 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued		105.0
Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 1 to Level 2 transfers, amount		$ 83.9				$ 83.9
Fair Value, Measurements, Recurring | Level 3 | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled			$ 87.4
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled	$ 111.2
Third party | Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt						$ 89.3

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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