Summary of Offsetting Assets and Liabilities - Netting Arrangements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial instruments (assets):
Gross fair value of recognized assets	$ 60,318	$ 52,738
Financial instruments	(49,741)	(47,827)
Cash collateral	(8,939)	(4,911)
Net fair value	1,638	0
Securities purchased under agreements to resell, gross		23,200
Securities purchased under agreements to resell, not offset		(23,200)
Securities purchased under agreements to resell, collateral, obligation to return cash		0
Securities purchased under agreements to resell, offset against collateral, net of not subject to master netting arrangement, policy election		0
Derivative asset, securities purchased under agreements to resell, securities borrowed		75,938
Derivative asset, securities purchased under agreements to resell, securities borrowed, not offset		(71,027)
Derivative asset, securities purchased under agreements to resell, securities borrowed, collateral, obligation to return cash		(4,911)
Derivative asset, securities purchased under agreements to resell, securities borrowed, amount offset against collateral		0
Financial instruments (liabilities):
Gross fair value of recognized liabilities	73,363	93,761
Financial instruments	(49,741)	(47,827)
Amount of unrestricted cash collateral to counterparties in the normal course of business	(20,588)	(42,750)
Net fair value	$ 3,034	$ 3,184

Fair value, after effects of master netting arrangements, deduction of assets not subject to a master netting arrangement and elected not to be offset, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, offset against an obligation to return collateral.

Name:	us-gaap_DerivativeAssetFairValueOffsetAgainstCollateralNetOfNotSubjectToMasterNettingArrangementPolicyElection
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be offset, deducted from derivative assets.