Investment Securities - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investment Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amount Outstanding	$ 2,924,407,000		$ 2,226,409,000
Fair Value	$ 2,922,504,000		$ 2,217,852,000
Average percent of amortized cost for securities in unrealized loss positions for more than 12 months	99.20%
Other-than-temporary impairment	$ 0	$ 0
Floating Interest Rate [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amount Outstanding	1,700,000
Fair Value	$ 1,700,000