Investments - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	$ 525,032	$ 572,915
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12,938	6,953
12 months or more, fair value	134,492	12,229
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6,087	589
Total, fair value	659,524	585,144
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	19,025	7,542
Obligations of U.S. Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	98,044	89,309
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,858
Securities Available for Sale, Continuous Unrealized Loss Position, Less Than 12 Months, Aggregate Losses		660
12 months or more, fair value	0	0
Securities Available for Sale, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, fair value	98,044	89,309
Securities Available for Sale Continuous Unrealized Loss Position, Total Aggregate Losses		660
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,858
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	24,883	13,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	369	259
12 months or more, fair value	364	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	25	0
Total, fair value	25,247	13,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	394	259
Agency mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	392,242	469,655
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10,617	6,034
12 months or more, fair value	134,128	12,229
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6,062	589
Total, fair value	526,370	481,884
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	16,679	$ 6,623
US Treasury Bill Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	9,863
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	94
12 months or more, fair value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, fair value	9,863
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 94