Investments (Unrealized Losses) (Details) $ in Thousands	Dec. 31, 2018 USD ($) investments investment	Dec. 31, 2017 USD ($)
Investment securities available for sale:
Less than 12 months, fair value	$ 705,526	$ 3,752,642
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	3,943	21,458
Aggregate fair value, continuous unrealized loss position, 12 months or more	2,796,627	2,975,631
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	52,489	59,470
Total fair value	3,502,153	6,728,273
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 56,432	80,928
Investment securities held to maturity:
Number of investments in continuous unrealized loss position for more than twelve months | investment	221
U. S. Treasury
Investment securities available for sale:
Less than 12 months, fair value	$ 248,983	1,408,166
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	113	345
Aggregate fair value, continuous unrealized loss position, 12 months or more	848,622	249,698
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,053	201
Total fair value	1,097,605	1,657,864
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 2,166	546
Investment securities held to maturity:
Number of investments in continuous unrealized loss position for more than twelve months | investments	7
Government Agency
Investment securities available for sale:
Less than 12 months, fair value	$ 115,273	848
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	601	12
Aggregate fair value, continuous unrealized loss position, 12 months or more	2,310	2,527
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	38	28
Total fair value	117,583	3,375
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	639	40
Mortgage Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	262,204	2,333,254
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	2,387	20,911
Aggregate fair value, continuous unrealized loss position, 12 months or more	1,940,695	2,723,406
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	50,376	59,241
Total fair value	2,202,899	5,056,660
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	52,763	80,152
Investment securities held to maturity:
Less than 12 months, fair value	5,111
Held-to-maturity Securities Continuous Unrealized Loss Position Less Than 12 Months Accumulated Loss	181
12 months or more, fair value	10,131
Held-to-maturity Securities Continuous Unrealized Loss Position 12 Months Or Longer Accumulated Loss	309
Total fair value	$ 15,242
Number of investments in continuous unrealized loss position for more than twelve months | investments	213
Corporate Bonds
Investment securities available for sale:
Less than 12 months, fair value	$ 79,066	5,025
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	842	8
Aggregate fair value, continuous unrealized loss position, 12 months or more	5,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22	0
Total fair value	84,066	5,025
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 864	8
Other Debt Obligations
Investment securities available for sale:
Less than 12 months, fair value		5,349
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments		182
Aggregate fair value, continuous unrealized loss position, 12 months or more		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Total fair value		5,349
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		$ 182