Condensed Financial Statements of Parent Company - Condensed balance sheet - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS.
Cash	$ 377,627	$ 374,448	$ 695,794	$ 417,869
Investment securities available for sale	1,650,710	999,405
Other Assets	126,590	72,943
Total assets	14,466,589	8,900,592
Liabilities.	12,157,669	7,766,004
Shareholders' equity	2,308,920	1,134,588	1,059,384	984,920
Total liabilities and shareholders' equity	14,466,589	8,900,592
Parent company
ASSETS.
Cash	25,879	27,775	$ 21,592	$ 13,185
Investment securities available for sale		631
Investment in subsidiaries	2,399,744	1,163,616
Other Assets	1,111	1,235
Total assets	2,426,734	1,193,257
Liabilities.	117,813	58,669
Shareholders' equity	2,308,921	1,134,588
Total liabilities and shareholders' equity	$ 2,426,734	$ 1,193,257