CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 43,314	$ 33,148
Other comprehensive income (loss):
Unrealized gain on debt securities on which an OTTI has been recognized	9	496
All other unrealized holding gain (losses) on available-for-sale securities arising during the period	20,501	(24,549)
Other comprehensive income (loss) for the period	20,510	(24,053)
Total comprehensive income	$ 63,824	$ 9,095