Investments - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Investment Grade Rate	96.60%
Fair value of issued securities	$ 7,475,565	$ 7,724,075
Federal Home Loan Bank Funding Agreements	625,000	625,000
Federal Home Loans Bank Of Chicago
Investment Holdings [Line Items]
Fair value of issued securities	625,165	686,790
Governmental Agencies as Required by Law in Various States
Investment Holdings [Line Items]
Fair value of issued securities	$ 17,782	$ 17,985

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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