Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 159,365	$ 152,716	$ 150,314
Other comprehensive income (loss):
Change in unrealized gains on securities	(26)	5,072	46,780
Less: reclassification adjustment for gains in net income	(10,038)	(29,673)	(23,529)
Increase (decrease) in fair value of cash flow hedge	884	(1,506)	(6,062)
Less: reclassification adjustment for amounts recognized in net income	2,627	3,928	4,498
Total other comprehensive income (loss)	(6,553)	(22,179)	21,687
Comprehensive income	152,812	130,537	172,001
Less: comprehensive income attributable to noncontrolling interest	0	(1,176)	(1,452)
Comprehensive income attributable to common stockholders	$ 152,812	$ 129,361	$ 170,549