Investments - Schedule of Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	$ 390,088	$ 572,915
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10,230	6,953
12 months or more, fair value	313,334	12,229
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	14,570	589
Total, fair value	703,422	585,144
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	24,800	7,542
Obligations of U.S. Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	58,592	89,309
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,353	660
12 months or more, fair value	39,089	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	880
Total, fair value	97,681	89,309
Securities Available for Sale Continuous Unrealized Loss Position, Total Aggregate Losses		660
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,233
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	24,152	13,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	468	259
12 months or more, fair value	3,296	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	124	0
Total, fair value	27,448	13,951
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	592	259
Agency mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	297,577	469,655
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	8,216	6,034
12 months or more, fair value	270,949	12,229
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	13,566	589
Total, fair value	568,526	481,884
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	21,782	$ 6,623
US Treasury Bill Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	9,767
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	193
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	9,767
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 193