Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 3,427,741	$ 3,122,950
Net unrealized (losses) gains arising during the period	$ 2,856	$ (4,424)	19,831	12,675
Amounts reclassified from AOCI	(892)	(1,038)	(3,902)	(4,908)
Other comprehensive income (loss)	1,964	(5,462)	15,929	7,767
Ending balance	3,781,896	3,378,054	3,781,896	3,378,054
Available-for-Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(18,950)	11,756	(28,772)	(6,144)
Net unrealized (losses) gains arising during the period	(1,014)	(3,869)	11,818	17,901
Amounts reclassified from AOCI	(892)	(1,038)	(3,902)	(4,908)
Other comprehensive income (loss)	(1,906)	(4,907)	7,916	12,993
Ending balance	(20,856)	6,849	(20,856)	6,849
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(15,231)	(13,468)	(19,374)	(8,797)
Net unrealized (losses) gains arising during the period	3,870	(555)	8,013	(5,226)
Amounts reclassified from AOCI	0	0	0	0
Other comprehensive income (loss)	3,870	(555)	8,013	(5,226)
Ending balance	(11,361)	(14,023)	(11,361)	(14,023)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(34,181)	(1,712)	(48,146)	(14,941)
Other comprehensive income (loss)			15,929	7,767
Ending balance	$ (32,217)	$ (7,174)	$ (32,217)	$ (7,174)