Securities, Securities in Continuous Unrealized Loss Position (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Security Institution Bond	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Securities in Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 48,407		$ 37,066
Less than 12 months, unrealized (losses)	(1,115)		(394)
12 months or more, fair value	9,590		11,548
12 months or more, unrealized (losses)	(418)		(431)
Total, fair value	57,997		48,614
Total, unrealized (losses)	$ (1,533)		(825)
Impaired Securities [Abstract]
Number of securities in a loss position | Security	80
Corporate bonds with cost basis net of other-than-temporary impairment	$ 70,817		72,198
Number of different financial institutions per bond | Institution	55
Estimated maturity	16 years
Anniversary on which securities could have been called at par	5 years
Frequency of repricing	3 months
Interest income	$ 6,370	$ 5,415
LIBOR [Member]
Impaired Securities [Abstract]
Term of variable rate	3 months
Obligations of U.S. Government Corporations and Agencies [Member]
Securities in Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 39,016		32,512
Less than 12 months, unrealized (losses)	(897)		(330)
12 months or more, fair value	9,590		10,008
12 months or more, unrealized (losses)	(418)		(278)
Total, fair value	48,606		42,520
Total, unrealized (losses)	(1,315)		(608)
Impaired Securities [Abstract]
Corporate bonds with cost basis net of other-than-temporary impairment	53,313		52,872
Obligations of States and Political Subdivisions [Member]
Securities in Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	9,013		4,172
Less than 12 months, unrealized (losses)	(208)		(60)
12 months or more, fair value	0		0
12 months or more, unrealized (losses)	0		0
Total, fair value	9,013		4,172
Total, unrealized (losses)	(208)		(60)
Impaired Securities [Abstract]
Corporate bonds with cost basis net of other-than-temporary impairment	14,754		15,124
Corporate Bonds [Member]
Securities in Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	0		0
Less than 12 months, unrealized (losses)	0		0
12 months or more, fair value	0		1,540
12 months or more, unrealized (losses)	0		(153)
Total, fair value	0		1,540
Total, unrealized (losses)	$ 0		(153)
Impaired Securities [Abstract]
Number of impaired securities | Security	2
Corporate bonds with cost basis net of other-than-temporary impairment	$ 2,362		3,816
Interest income	$ 50	$ 32
Corporate Bonds [Member] | Performing [Member]
Impaired Securities [Abstract]
Bonds classified as performing | Bond	2
Mutual Funds [Member]
Securities in Continuous Unrealized Loss Position [Abstract]
Less than 12 months, fair value	$ 378		382
Less than 12 months, unrealized (losses)	(10)		(4)
12 months or more, fair value	0		0
12 months or more, unrealized (losses)	0		0
Total, fair value	378		382
Total, unrealized (losses)	(10)		(4)
Impaired Securities [Abstract]
Corporate bonds with cost basis net of other-than-temporary impairment	388		$ 386
1 Corporate Bond [Member]
Impaired Securities [Abstract]
Face value of corporate bond	2,000
Gain realized on corporate bond due to auction and settlement	$ 535