Securities Fair Value Option Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Option Securities [Line Items]
Fair value option securities, Fair Value	$ 1,816,398	$ 283,235
Fair value option securities, Net Unrealized Gain (Loss)	19,515	2,766
U.S. Treasury [Member]
Fair Value Option Securities [Line Items]
Fair value option securities, Fair Value	552,536	0
Fair value option securities, Net Unrealized Gain (Loss)	927	0
Residential agency mortgage-backed securities [Member]
Fair Value Option Securities [Line Items]
Fair value option securities, Fair Value	1,263,862	283,235
Fair value option securities, Net Unrealized Gain (Loss)	$ 18,588	$ 2,766