Fair Value Disclosures - Level 3 Roll Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Net Asset (Liability) Value [Roll Forward]
Balance at December 31, 2016	$ 30,907	$ 0
Purchases/additions	(430)	2,211
Sales	0	0
Payments received	(137)	(65)
Premium amortization, net	0	0
(Decrease) increase in value	(432)	(17)
Balance at December 31, 2017	29,908	30,907
Derivative Financial Instruments, Liabilities [Member] | Credit Risk Contract [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2016	(9)	0
Purchases/additions	(430)	(102)
Sales	0	0
Payments received	0	0
Premium amortization, net	0	0
(Decrease) increase in value	403	93
Balance at December 31, 2017	(36)	(9)
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2016	28,778	0
Purchases/additions	0	0
Sales	0	0
Payments received	0	0
Premium amortization, net	0	0
(Decrease) increase in value	(792)	0
Balance at December 31, 2017	27,986	28,778
Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2016	2,138	0
Purchases/additions	0	2,313
Sales	0	0
Payments received	(137)	(65)
Premium amortization, net	0	0
(Decrease) increase in value	(43)	(110)
Balance at December 31, 2017	$ 1,958	$ 2,138