Debt - Narrative (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) subsidiary_business_trusts classes	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Securities sold under agreements to repurchase \| $	$ 65.5	$ 66.6
Number of classes of financial instruments \| classes	2
Number of business subsidiary trusts \| subsidiary_business_trusts	2
ACST III \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Junior Subordinated Debt, Preferred Securities \| ACST II \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	3.15%

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
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Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Name:	us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant