Acquired Loans - Changes in Accretable Yield of Acquired Loans (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Change in accretable difference on acquired loans [Abstract]
Accretable yield at beginning of period		$ (17,722)	$ (31,426)	$ (31,426)
Accretion to interest income		1,563	3,268
Disposals, net		469	543
Reclassification from nonaccretable difference	[1]	(1,725)	(1,353)
Accretable yield at end of period		$ (17,415)	$ (28,968)	$ (17,722)

[1] Reclassifications from nonaccretable difference are due to lower loss expectations and improvements in expected cash flows.